Browse practical how-to articles, formulas and worked examples connected directly to the calculators on ERICH Calculators.
Each cluster links practical search questions to the calculator that solves them.
Learn the break-even formula using fixed costs, selling price and variable cost per unit. See formulas, worked examples and related free calculators.
Read guide →Business Profitability & PricingLearn the difference between markup and profit margin with formulas and a pricing example. See formulas, worked examples and related free calculators.
Read guide →Business Profitability & PricingLearn how to calculate profit margin from revenue and expenses. See formulas, worked examples and related free calculators.
Read guide →Compare approximate bag counts for 40 lb, 60 lb and 80 lb premix concrete using planning yields, then verify the manufacturer coverage before purchasing.
Read guide →Concrete & MasonrySee a worked concrete-volume example for a 10-foot by 20-foot slab that is 4 inches thick. See formulas, worked examples and related free calculators.
Read guide →Concrete & MasonryA 12 ft by 12 ft slab at 4 inches thick is about 1.78 cubic yards before waste. See the cubic-foot, cubic-yard and bag calculations.
Read guide →Concrete & MasonryEstimate 80 lb concrete bags from cubic feet or cubic yards. One 80 lb bag is often approximated at 0.60 cubic feet of mixed concrete; verify the product label.
Read guide →Concrete & MasonryLearn how to calculate concrete volume for slabs and rectangular pours, convert cubic feet to cubic yards, and add a practical waste allowance.
Read guide →Concrete & MasonryLearn how to calculate cubic yards of concrete for a rectangular footing using length, width and depth.
Read guide →Learn how date-difference calculations work and why inclusive versus exclusive counting can change the answer.
Read guide →Dates & DeadlinesLearn how to add or subtract calendar days from a date across month and year boundaries. See formulas, worked examples and related free calculators.
Read guide →Dates & DeadlinesLearn how to count business days between dates, including 3–5 and 7–10 business-day examples, weekends, holidays and inclusive counting.
Read guide →See why 15,000 is a strong estimate for 26 × 604 and how rounding makes the multiplication easy.
Read guide →Estimation & RoundingEstimate 3920 minus 1731 by rounding to hundreds and checking the size of the answer.
Read guide →Estimation & RoundingEstimate 42.3 divided by 3.8 using compatible numbers and a quick reasonableness check.
Read guide →Estimation & RoundingEstimate 45.1 times 1.5 by rounding 45.1 to 45 and using one-and-a-half as a simple multiplier.
Read guide →Estimation & RoundingEstimate 862 − 421 quickly by rounding to convenient tens or hundreds.
Read guide →Estimation & RoundingLearn how to estimate multiplication problems by rounding one or both factors to friendly numbers.
Read guide →Estimation & RoundingLearn how to estimate division problems by rounding the dividend and divisor to compatible numbers.
Read guide →Estimation & RoundingLearn how to estimate addition and subtraction by rounding numbers to convenient place values.
Read guide →Learn how to estimate tile count from project area, tile dimensions and a waste allowance. See formulas, worked examples and related free calculators.
Read guide →Flooring, Tile & PaintLearn how to calculate room area and add a waste allowance when buying flooring. See formulas, worked examples and related free calculators.
Read guide →Flooring, Tile & PaintLearn how to estimate wall area, number of coats and gallons of paint. See formulas, worked examples and related free calculators.
Read guide →Learn how to add or subtract fractions by finding a common denominator and simplifying the result. See formulas and worked examples.
Read guide →Fractions, Ratios & ProportionsLearn how to turn a terminating decimal into a fraction using place value and reducing to lowest terms.
Read guide →Fractions, Ratios & ProportionsLearn how to solve proportions with cross multiplication and find an unknown value in two equal ratios.
Read guide →Learn the triangle perimeter formula, see a worked example and check whether three side lengths can form a valid triangle.
Read guide →Geometry & TrianglesFind a missing leg of a right triangle when the hypotenuse and the other leg are known.
Read guide →Geometry & TrianglesFind a missing side of a right triangle using the Pythagorean theorem and the two side lengths you already know.
Read guide →Geometry & TrianglesFind an unknown acute angle in a right triangle using sine, cosine or tangent.
Read guide →Geometry & TrianglesLearn the slope formula from two coordinate points and recognize positive, negative, zero and undefined slope.
Read guide →Geometry & TrianglesLearn the base-times-height formula for triangle area and how to use perpendicular height correctly.
Read guide →Geometry & TrianglesLearn the hypotenuse formula and calculate the longest side of a right triangle from the two legs.
Read guide →Geometry & TrianglesLearn how to use a²+b²=c² to find the hypotenuse or a missing leg of a right triangle. See formulas, worked examples and related free calculators.
Read guide →Geometry & TrianglesCalculate the area of a right triangle by multiplying the two perpendicular legs and dividing by two.
Read guide →Geometry & TrianglesUnderstand the legs, hypotenuse and side relationships in a right triangle before choosing a formula.
Read guide →Calculate the score needed when a final exam is worth 20% of your course grade, with the formula and worked examples for common targets.
Read guide →GPA, Grades & Test ScoresUse the 25%-final formula to find the exam score needed for your target course grade, with a worked 82% to 85% example.
Read guide →GPA, Grades & Test ScoresLearn how to calculate an unweighted GPA, how credit hours affect GPA, and how weighted GPA can differ for honors or advanced courses.
Read guide →GPA, Grades & Test ScoresLearn the difference between weighted and unweighted GPA and how honors or AP courses can change a weighted average.
Read guide →GPA, Grades & Test ScoresFind the final-exam score needed to reach a passing course grade using your current average, final weight and the passing cutoff for your class.
Read guide →GPA, Grades & Test ScoresLearn the final-exam grade formula and calculate the score needed to reach a target course grade. See formulas and worked examples.
Read guide →Learn the standard adult BMI ranges and how to interpret underweight, healthy weight, overweight and obesity categories.
Read guide →Health & Body MetricsCalculate adult BMI from weight in kilograms and height in meters using the metric formula.
Read guide →Health & Body MetricsCalculate adult BMI from weight in pounds and height in inches using the standard U.S. formula.
Read guide →Health & Body MetricsLearn the difference between basal metabolic rate and an activity-adjusted daily calorie estimate. See formulas and worked examples.
Read guide →Health & Body MetricsCompare body-fat percentage and body mass index and learn what each estimate can and cannot tell you.
Read guide →Health & Body MetricsLearn how healthy-weight estimates are commonly derived from height and why different formulas can produce different answers.
Read guide →Health & Body MetricsLearn the adult BMI formula using height and weight and why BMI is a screening estimate rather than a diagnosis.
Read guide →Health & Body MetricsLearn a simple age-based method for estimating maximum heart rate and percentage-based exercise zones.
Read guide →Health & Body MetricsEstimate maximum heart rate from age using a common age-based formula and understand its limitations.
Read guide →Health & Body MetricsSee example maximum-heart-rate estimates by age and learn how the 220 minus age rule changes over time.
Read guide →Health & Body MetricsLearn a simple body-weight-based way to estimate daily protein intake and how calorie targets and goals can affect the number.
Read guide →Health & Body MetricsLearn a simple way to estimate daily water intake from body weight and why activity level and climate can change the number.
Read guide →Calculate the percentage of gross monthly income used for housing expenses and understand what costs may be included.
Read guide →Home Affordability, Refinance & Rent vs. BuySee how income, monthly debt, down payment, mortgage rate and debt-to-income assumptions combine in a home-affordability estimate.
Read guide →Home Affordability, Refinance & Rent vs. BuyLearn how to compare refinance closing costs with monthly savings and estimate the number of months needed to break even.
Read guide →Home Affordability, Refinance & Rent vs. BuyLearn how a mortgage payment can be compared with monthly income as a quick housing-affordability check.
Read guide →Home Affordability, Refinance & Rent vs. BuyLearn the main costs to compare when deciding whether renting or buying is financially better over a chosen time horizon.
Read guide →Learn how to calculate gravel volume in cubic yards and estimate tons from area, depth and density. See formulas and worked examples.
Read guide →Landscaping & Outdoor MaterialsLearn how to calculate mulch cubic yards from landscape-bed area and desired depth. See formulas, worked examples and related free calculators.
Read guide →Landscaping & Outdoor MaterialsLearn how to calculate topsoil volume from area and depth and convert to cubic yards. See formulas, worked examples and related free calculators.
Read guide →See how a down payment reduces the amount financed and changes an estimated monthly auto-loan payment.
Read guide →Loans & Debt PayoffLearn how to calculate debt-to-income ratio and what monthly debt and income figures belong in the formula.
Read guide →Loans & Debt PayoffLearn how balance, APR and fixed monthly payment determine credit-card payoff time when no new charges are added.
Read guide →Loans & Debt PayoffLearn the fixed installment-loan payment formula and see how principal, APR and loan term affect monthly payments.
Read guide →Loans & Debt PayoffCompare a loan payment or loan balance with income to understand how repayment burden is often evaluated.
Read guide →Learn the difference between customer acquisition cost, cost per acquisition and customer lifetime value.
Read guide →Marketing Metrics & Customer EconomicsLearn the formulas for cost per click, cost per thousand impressions and click-through rate. See formulas, worked examples and related free calculators.
Read guide →Marketing Metrics & Customer EconomicsLearn the conversion-rate formula for websites, ads and funnels. See formulas, worked examples and related free calculators.
Read guide →Compare true biweekly mortgage payments with adding a fixed amount to each monthly payment. The timing and annual extra principal are not always the same.
Read guide →Mortgage Payments & AmortizationEstimate the payoff and interest effect of adding $200 per month to mortgage principal. Use your own balance, rate and remaining term in the calculator.
Read guide →Mortgage Payments & AmortizationLearn why applying extra money to principal can reduce future interest and shorten a mortgage payoff schedule.
Read guide →Mortgage Payments & AmortizationLearn how mortgage amortization splits each payment between principal and interest, why the split changes, and how extra payments can shorten payoff time.
Read guide →Mortgage Payments & AmortizationUse the fixed-rate mortgage payment formula to calculate monthly principal and interest from the loan amount, interest rate and term, with a worked example.
Read guide →Mortgage Payments & AmortizationCompare practical ways to shorten a mortgage payoff schedule, including extra principal and biweekly-style strategies.
Read guide →Mortgage Payments & AmortizationCompare a one-time mortgage principal payment with smaller recurring extra payments.
Read guide →Mortgage Payments & AmortizationEstimate principal-and-interest payment on a $250,000 30-year mortgage at a 6% annual interest rate.
Read guide →Mortgage Payments & AmortizationSee how an extra $100 monthly principal payment can shorten a mortgage and reduce interest, with an example and a link to the payoff calculator.
Read guide →Mortgage Payments & AmortizationSee a worked example for a $300,000 30-year mortgage at 6.5%, including monthly principal and interest and lifetime interest.
Read guide →Quick conversion formulas and charts for length, weight, temperature, volume, speed and energy, with worked examples and free converters.
Read guide →Pace, Speed & Unit ConversionsLearn how to calculate running pace per mile or kilometer from total time and distance. See formulas, worked examples and related free calculators.
Read guide →Pace, Speed & Unit ConversionsLearn the direct formula for converting miles per hour into minutes per mile. See formulas, worked examples and related free calculators.
Read guide →Learn how to calculate 15% off any price. Multiply by 0.15 for the discount, or multiply by 0.85 for the final sale price.
Read guide →Percentages, Discounts & Sales Tax20% off $100 saves you $20, making the final price $80. See the calculation and use our discount calculator for any price or percentage.
Read guide →Percentages, Discounts & Sales TaxLearn the percentage decrease formula with worked examples, denominator rules, percentage-point differences and an Excel or Google Sheets formula.
Read guide →Percentages, Discounts & Sales TaxUse the percentage increase formula with worked price, salary and number examples. Learn which value goes in the denominator and when percent change is undefined.
Read guide →Percentages, Discounts & Sales TaxLearn the sales-tax formula, calculate tax and total price, reverse sales tax, and see examples for common tax rates and discounts.
Read guide →Percentages, Discounts & Sales TaxFind the original whole when you know a result and the percentage. Example: if 45 is 30% of a number, the original number is 150.
Read guide →Learn how to convert a multi-year investment gain into an equivalent annual compound return. See formulas, worked examples and related free calculators.
Read guide →ROI & Investment ReturnsLearn the basic ROI formula, see examples of investment and business ROI, and understand the limits of ROI when time, risk and cash flows differ.
Read guide →ROI & Investment ReturnsCompare total return on investment with compound annual growth rate. See formulas, worked examples and related free calculators.
Read guide →Learn how double-time pay works in simple arithmetic terms and how it differs from regular pay and time-and-a-half.
Read guide →Salary, Pay & OvertimeEstimate how recurring overtime can change the annual gross-pay equivalent of an hourly job.
Read guide →Salary, Pay & OvertimeCalculate a pay raise percentage, new salary or new hourly wage. See formulas for old pay vs. new pay and worked hourly and annual examples.
Read guide →Salary, Pay & OvertimeLearn how to calculate gross pay before taxes and deductions using hourly pay, salary equivalents and overtime adjustments.
Read guide →Salary, Pay & OvertimeLearn how to estimate overtime pay from regular hours, overtime hours, hourly rate and an overtime multiplier.
Read guide →Salary, Pay & OvertimeLearn how to convert hourly pay to weekly, monthly and annual gross salary estimates. See formulas, worked examples and related free calculators.
Read guide →Salary, Pay & OvertimeLearn how to estimate an hourly rate from weekly pay by dividing weekly gross pay by hours worked.
Read guide →Salary, Pay & OvertimeEstimate gross weekly pay from an hourly wage, regular hours and overtime hours.
Read guide →Salary, Pay & OvertimeLearn the basic arithmetic behind overtime after 40 hours in a workweek and how regular pay and overtime pay are combined.
Read guide →Salary, Pay & OvertimeSee how regular pay and time-and-a-half overtime combine when the regular hourly rate is $20.
Read guide →Salary, Pay & OvertimeSee how to calculate time-and-a-half overtime when the regular hourly rate is $25.
Read guide →Salary, Pay & OvertimeLearn how to calculate time-and-a-half pay by multiplying the regular hourly rate by 1.5 and applying that rate to overtime hours.
Read guide →Compare simple and compound interest, learn both formulas, and see why compounding changes long-term growth.
Read guide →Savings, Interest & GrowthLearn the APY formula, convert a nominal interest rate to effective annual yield, and see how compounding frequency changes savings or CD returns.
Read guide →Savings, Interest & GrowthSee a worked compound-interest example showing how $10,000 grows at 5% for 10 years with annual compounding.
Read guide →Learn the z-score formula and how to interpret distance from the mean in standard-deviation units. See formulas and worked examples.
Read guide →Statistics & ProbabilityLearn what standard deviation measures, how population and sample standard deviation differ, and how to calculate variance and standard deviation step by step.
Read guide →Statistics & ProbabilityLearn the difference between mean, median and mode and when each measure of center is most useful. See formulas and worked examples.
Read guide →Learn how a one-hour unpaid lunch changes common 8-to-5 and 9-to-6 schedules.
Read guide →Work Hours & Time CardsA 30-minute unpaid lunch equals 0.50 decimal hour. See how to subtract it correctly from a time card.
Read guide →Work Hours & Time CardsSee how lunch length affects the clock-in and clock-out times needed to reach 40 paid hours over five days.
Read guide →Work Hours & Time CardsA 45-minute unpaid lunch equals 0.75 decimal hour. See the conversion and time-card examples.
Read guide →Work Hours & Time CardsSee schedule examples that produce 7.5 paid hours and learn why 7 hours 30 minutes equals 7.50 decimal hours.
Read guide →Work Hours & Time CardsSee common clock-in, clock-out and lunch combinations that produce exactly 8 paid hours.
Read guide →Work Hours & Time Cards8:00 AM to 4:30 PM is 8.5 elapsed hours. Subtract a 30-minute unpaid lunch and paid time is exactly 8 hours. Compare other lunch lengths.
Read guide →Work Hours & Time Cards9:00 AM to 5:00 PM is 8 elapsed hours. Subtract a 30-minute unpaid lunch and paid time is 7.5 hours, or 7 hours 30 minutes.
Read guide →Work Hours & Time CardsConvert common minute values to decimal hours for payroll and time-card calculations.
Read guide →Work Hours & Time CardsLearn how to find elapsed hours between start and end times, including breaks and overnight shifts. See formulas and worked examples.
Read guide →Work Hours & Time CardsCalculate hours when a shift starts before midnight and ends the next day, including unpaid breaks.
Read guide →Work Hours & Time CardsLearn how to calculate work hours from clock-in and clock-out times, subtract unpaid breaks, convert minutes to decimal hours, and total a workweek.
Read guide →Work Hours & Time CardsLearn how to subtract unpaid lunch minutes from a shift and convert the result to decimal hours.
Read guide →Work Hours & Time CardsLearn how to convert payroll-style decimal hours such as 7.75 into 7 hours 45 minutes. See formulas, worked examples and related free calculators.
Read guide →Work Hours & Time CardsAdd all unpaid break minutes together, then subtract the total from elapsed work time.
Read guide →Work Hours & Time CardsUnderstand when lunch time is included in paid hours and how to calculate a schedule either way.
Read guide →Work Hours & Time CardsLearn how to subtract a 30-, 45- or 60-minute unpaid lunch from clock-in and clock-out times and convert the result to decimal hours.
Read guide →Work Hours & Time CardsQuick-reference chart for common work schedules, lunch lengths and paid decimal hours.
Read guide →