Learn the formulas, methods and common mistakes behind popular calculations. The library is organized into 20 topic clusters so related guides and calculators reinforce one another.
Mortgage payment formulas, amortization schedules, extra principal and worked loan-payment scenarios.
Learn how mortgage amortization works, why early payments contain more interest, and how extra payments can change total interest and payoff time.
Read guide →Mortgage Payments & AmortizationLearn the fixed-rate mortgage payment formula and how loan amount, rate and term affect the monthly principal-and-interest payment.
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 →Compare how income, debt, rates, refinancing costs and ownership assumptions affect housing decisions.
Learn how income, debts, down payment, mortgage rate and debt-to-income ratio 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 the main costs to compare when deciding whether renting or buying is financially better over a chosen time horizon.
Read guide →Payment formulas, credit-card payoff time, debt ratios and installment-loan planning.
Learn the fixed installment-loan payment formula and see how principal, APR and loan term affect monthly payments.
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 how to calculate debt-to-income ratio and what monthly debt and income figures belong in the formula.
Read guide →Compound interest, APY, future value and long-term savings-growth calculations.
Compare simple and compound interest, learn both formulas, and see why compounding changes long-term growth.
Read guide →Savings, Interest & GrowthLearn the APY formula and how compounding frequency converts a nominal rate into an effective annual yield.
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 →Convert hourly pay and salary, estimate overtime and understand percentage pay raises.
Learn how to convert hourly pay to weekly, monthly and annual gross salary estimates.
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 calculate a new salary or hourly wage after a percentage raise.
Read guide →Percentage change, discounts, tax, tips and everyday percent-of-number questions.
Learn the percentage increase and decrease formulas, see worked examples, and avoid common mistakes when comparing an old value with a new value.
Read guide →Percentages, Discounts & Sales TaxSee the formula for a 20% discount on $100 and learn how to calculate any discounted sale price.
Read guide →Percentages, Discounts & Sales TaxLearn the sales-tax formula and how to calculate the tax amount and final purchase total.
Read guide →Fraction arithmetic, decimal-to-fraction conversion, ratios and proportions.
Learn how to add or subtract fractions by finding a common denominator and simplifying the result.
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 →Averages, standard deviation, z-scores, normal distributions and probability calculations.
Learn 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.
Read guide →Statistics & ProbabilityLearn the z-score formula and how to interpret distance from the mean in standard-deviation units.
Read guide →Right triangles, areas, slopes, distances and common coordinate-geometry formulas.
Learn how to use a²+b²=c² to find the hypotenuse or a missing leg of a right triangle.
Read guide →Geometry & TrianglesLearn the base-times-height formula for triangle area and how to use perpendicular height correctly.
Read guide →Geometry & TrianglesLearn the slope formula from two coordinate points and recognize positive, negative, zero and undefined slope.
Read guide →Calendar-day differences, business-day counting, days-until questions and date arithmetic.
Learn how to count business days between two dates, how weekends and holidays affect the answer, and when inclusive or exclusive counting changes the result.
Read guide →Dates & DeadlinesLearn 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.
Read guide →Clock-in and clock-out math, unpaid breaks, decimal hours and weekly time-card totals.
Learn 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 convert payroll-style decimal hours such as 7.75 into 7 hours 45 minutes.
Read guide →Work Hours & Time CardsLearn how to find elapsed hours between start and end times, including breaks and overnight shifts.
Read guide →GPA, weighted grades, final-exam targets and test-score calculations.
Learn 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 final-exam grade formula and calculate the score needed to reach a target course grade.
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 →Concrete volume, slabs, footings, blocks, bricks, pavers and related material quantities.
Learn 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 & MasonrySee a worked concrete-volume example for a 10-foot by 20-foot slab that is 4 inches thick.
Read guide →Concrete & MasonryLearn how to calculate cubic yards of concrete for a rectangular footing using length, width and depth.
Read guide →Floor area, tile quantities, paint coverage and interior finishing material estimates.
Learn how to calculate room area and add a waste allowance when buying flooring.
Read guide →Flooring, Tile & PaintLearn how to estimate tile count from project area, tile dimensions and a waste allowance.
Read guide →Flooring, Tile & PaintLearn how to estimate wall area, number of coats and gallons of paint.
Read guide →Gravel, mulch, topsoil, sand, lawn and outdoor project material estimates.
Learn how to calculate gravel volume in cubic yards and estimate tons from area, depth and density.
Read guide →Landscaping & Outdoor MaterialsLearn how to calculate mulch cubic yards from landscape-bed area and desired depth.
Read guide →Landscaping & Outdoor MaterialsLearn how to calculate topsoil volume from area and depth and convert to cubic yards.
Read guide →Markup, margin, break-even and profit calculations for pricing and operating decisions.
Learn the difference between markup and profit margin with formulas and a pricing example.
Read guide →Business Profitability & PricingLearn the break-even formula using fixed costs, selling price and variable cost per unit.
Read guide →Business Profitability & PricingLearn how to calculate profit margin from revenue and expenses.
Read guide →CPC, CPM, CTR, conversion rate, acquisition cost and customer-value calculations.
Learn the formulas for cost per click, cost per thousand impressions and click-through rate.
Read guide →Marketing Metrics & Customer EconomicsLearn the conversion-rate formula for websites, ads and funnels.
Read guide →Marketing Metrics & Customer EconomicsLearn the difference between customer acquisition cost, cost per acquisition and customer lifetime value.
Read guide →Total ROI, annualized return, CAGR and investment-performance comparisons.
Learn 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.
Read guide →ROI & Investment ReturnsLearn how to convert a multi-year investment gain into an equivalent annual compound return.
Read guide →BMI, BMR, calorie needs, body measurements and heart-rate estimates for general reference.
Learn the adult BMI formula using height and weight and why BMI is a screening estimate rather than a diagnosis.
Read guide →Health & Body MetricsLearn the difference between basal metabolic rate and an activity-adjusted daily calorie estimate.
Read guide →Health & Body MetricsLearn a simple age-based method for estimating maximum heart rate and percentage-based exercise zones.
Read guide →Running and walking pace, speed relationships and common everyday unit-conversion formulas.
Learn how to calculate running pace per mile or kilometer from total time and distance.
Read guide →Pace, Speed & Unit ConversionsLearn the direct formula for converting miles per hour into minutes per mile.
Read guide →Pace, Speed & Unit ConversionsUse a quick reference for common length, weight, temperature and volume conversions.
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