Calculator Guides
Straight answers to the questions behind our calculators, with the formula, a worked example and the official sources. RSS feed
Financial
- 15 vs. 30-Year Mortgage: Which Is Better?Compare a 15-year and 30-year mortgage on a $320,000 loan: monthly payment, total interest, equity after 5 and 10 years, and when the shorter term is worth it.
- APR vs APY: What's the Difference?APR is the yearly rate before compounding; APY includes it. See the effective annual rate formula and how a 24% APR becomes 27.12% with daily interest.
- Compound Interest Formula, With Examples and the Rule of 72The compound interest formula A = P(1 + r/n)^(nt) explained step by step, with a compounding-frequency table, worked examples and the rule of 72 for doubling.
- How Are RMDs Calculated? The IRS Formula With ExamplesThe RMD formula: last December 31 balance divided by your IRS Uniform Lifetime Table factor. Worked examples, divisors by age, deadlines and the 25% penalty.
- How Is GDP Calculated?How gross domestic product is calculated with C + I + G + (X − M), the income and production approaches, and how real GDP and the GDP deflator remove inflation.
- How Is Social Security Calculated?How Social Security is calculated: indexed earnings, AIME, the 90/32/15 PIA formula with 2026 bend points, and how claiming from 62 to 70 changes your check.
- How Much House Can I Afford?How much house you can afford on your income using the 28/36 rule, with a price table for $50k to $200k salaries, the effect of debts, rates and down payment.
- How Much Should I Save for Retirement?Save 10% to 15% of income and aim for about 25 times yearly spending (the 4% rule). See savings rates by starting age, a worked example and 2026 limits.
- How to Calculate a Mortgage PaymentThe mortgage payment formula M = P × r(1+r)^n ÷ ((1+r)^n − 1) explained step by step, with a $320,000 worked example, a rate table and how taxes and PMI add on.
- How to Calculate Take-Home PayCalculate take-home pay step by step: subtract pre-tax deductions, 2026 federal tax, Social Security, Medicare and state tax, with a worked $75,000 example.
- How to Convert Hourly Pay to Annual SalaryConvert hourly pay to salary: multiply the rate by 2,080 hours. $25 an hour is $52,000 a year. Includes a $15 to $60 an hour table and overtime math.
- How to Pay Off Credit Card DebtHow to pay off credit card debt faster: payoff times for a $6,000 balance at 22%, avalanche vs snowball on three cards, and when consolidation saves money.
- Profit Margin vs Markup: What's the Difference?Profit margin is profit divided by price; markup is profit divided by cost. See both formulas, gross vs net margin, and a table to convert between them.
- Roth IRA vs Traditional IRA: Which Should You Choose?Roth or traditional IRA? Compare tax now vs later with the break-even formula, a 35-year worked example, 2026 contribution and income limits, and RMD rules.
- Should I Rent or Buy a Home?Rent or buy comes down to how long you stay. See a $400,000 home vs. $2,400 rent run through our calculator, with break-even years for rates, rents and prices.
Fitness & Health
- How Many Calories Should I Eat to Lose Weight?Use the calorie deficit formula: maintenance (TDEE) minus 250 to 1,000 calories a day. Worked examples, a target table, safe minimums and safe rates of loss.
- How Much Protein Do I Need a Day?How many grams of protein to eat a day: the 0.8 g/kg RDA, higher targets for athletes, older adults and pregnancy, and a body-weight table from 120 to 250 lb.
- How Much Water Should I Drink a Day?About 3.0 liters of drinks a day for men and 2.2 for women, per National Academies figures. See a body-weight table, exercise extras and weight-loss advice.
- How to Calculate Macros: Formula and ExamplesCalculate macros by multiplying daily calories by each share, then dividing by 4 for protein and carbs and 9 for fat. Worked weight-loss examples and ranges.
- How to Calculate Your Due DateCalculate your due date with Naegele's rule (LMP + 280 days), from a conception or IVF transfer date, or from an early ultrasound, with worked examples.
- How to Measure Body Fat PercentageMeasure body fat at home with a tape and the US Navy formula, see healthy body fat ranges for men and women, and compare DEXA, calipers and smart scales.
- TDEE vs BMR: Formulas and the Difference ExplainedBMR is the calories you burn at rest; TDEE adds activity. See the Mifflin-St Jeor and Harris-Benedict formulas, activity multipliers and worked examples.
- What Is a Healthy BMI?A healthy adult BMI is 18.5 to 24.9. See the BMI formula in pounds and kilograms, healthy weights by height, child percentiles and where BMI falls short.
- What Is My Target Heart Rate by Age?Estimate max heart rate with 220 minus age or Tanaka, then find your target zone. Chart by age, the Karvonen method and how the 5 training zones compare.
- When Am I Ovulating?Ovulation comes about 14 days before your next period. See the formula, your fertile window by cycle length, body signs to watch and why dates are estimates.
Math
- Area Formulas for Common ShapesArea formulas for the square, rectangle, triangle, circle, parallelogram and trapezoid, with a worked example for each and how to convert square units.
- How to Add FractionsHow to add fractions with unlike denominators using the formula and the LCD, plus how to subtract, multiply, divide, simplify and work with mixed numbers.
- How to Calculate an AverageHow to calculate an average, with the weighted average and average rate of change formulas, mean vs median vs mode, and when the median is the better choice.
- How to Calculate PercentagesThe percentage formula for X% of Y, X as a percent of Y and percent change, plus how to take 20% off a price and why percentage points differ from percent.
- How to Calculate Standard DeviationThe standard deviation formula for a population (divide by n) and a sample (divide by n − 1), worked step by step on eight numbers with a full table.
- How to Solve Equations: Solving for x Step by StepSolve for x step by step: linear equations, equations with fractions, systems of two equations by elimination or substitution, and when to use another method.
- Triangle Area Formula and Other Triangle FormulasTriangle area formulas (½bh and Heron's), the Pythagorean theorem, triangle types, the laws of sines and cosines, and common right triangles, with examples.
- Volume Formulas for Common 3D ShapesVolume formulas for the cube, box, cylinder, cone, pyramid and sphere, with worked examples, surface area, and conversions to gallons, liters and cubic yards.
- What Is the Distance Formula?The distance formula d = √((x₂ − x₁)² + (y₂ − y₁)²) with worked 2D and 3D examples, its link to the Pythagorean theorem, and the midpoint formula.
- What Is the Probability Formula?The probability formula with worked dice and card examples, plus the complement, AND and OR rules, conditional and binomial probability, and nPr vs nCr.
- What Is the Quadratic Formula?The quadratic formula x = (−b ± √(b² − 4ac)) ÷ 2a with worked examples, what the discriminant tells you, how complex roots work, and when factoring is faster.
- What Is the Slope Formula?The slope formula m = (y₂ − y₁) ÷ (x₂ − x₁) with worked examples, slope-intercept and point-slope forms, undefined slope, and slope as a percent grade.
Other
- How Much Concrete Do I Need?The concrete formula for slabs, footings and columns, with cubic yards worked out, bags per yard for 40, 60 and 80 lb sizes, and a 5 to 10 percent allowance.
- How Much Mulch Do I Need?Find mulch in cubic yards with square feet × depth in inches ÷ 324. Includes a bag chart for 2 and 3 cu ft bags, recommended depths and gravel and concrete.
- How Much Tile Do I Need?Work out tile for a room: measure square feet, find tiles per square foot by size, allow for grout joints and waste, and round up to boxes, with an example.
- How to Calculate Age From a Date of BirthThe age formula in years, months and days, how to borrow when a subtraction goes negative, how February 29 birthdays count, and the DATEDIF method.
- How to Calculate Gas MileageThe gas mileage formula from one fill-up to the next, how to convert MPG to L/100 km, and how to work out fuel cost per mile and trip cost, with examples.
- How to Calculate GPAThe GPA formula on a 4.0 scale with a worked example, how credits change the result, cumulative GPA and CGPA, and how weighted and unweighted GPA differ.
- How to Calculate Hours WorkedCalculate hours worked from clock times: convert minutes to decimals, subtract unpaid breaks, handle overnight shifts, 15-minute rounding and FLSA overtime.
- How to Calculate Roof SquaresOne roofing square is 100 square feet. Find your roof's squares from its footprint and pitch multiplier, then work out shingle bundles and a waste allowance.
- How to Calculate Square FootageMultiply length by width in feet to find square footage. Worked examples for rooms, L-shapes, inches, circles and triangles, plus square feet in an acre.
- How to Convert Feet to CMConvert feet and inches to cm with the exact 2.54 factor, convert cm back to feet, and read heights from 4'10" to 6'6" in a chart, with advice on rounding.
- How to Count the Days Between Two DatesCount the days between two dates by hand or with a formula: month lengths, leap years, including the end date, business days, and spreadsheet methods.
- Resistor Color Code Chart and How to Read ItA full 4-band and 5-band resistor color code chart, worked examples, what tolerance means, and the series and parallel resistor formulas, with Ohm's law.
- Speed, Distance and Time FormulaThe formula for speed and its two rearrangements, a mph, km/h and m/s conversion table, and why average speed is not the average of two speeds, with an example.