Resources & How-To Guides
Short, practical guides that show the math behind our tools — with real examples you can follow. Read the guide, then let the matching calculator do the work.
Home & GardenHow much paint do I need for a room?Work out how much paint a room needs in gallons or liters: a wall-area formula, a worked 12x12 example at about 2 gallons (7.5 L), coverage figures, and mistakes that waste paint.Read guideFinanceHow long will it take to pay off a credit card?How long to pay off a credit card: the payoff formula, why interest slows you down, and a worked 5,000 dollar example at 22 percent APR. Estimates only.Read guideConstructionHow much concrete do I need for a slab?Calculate concrete for a slab in cubic yards or cubic meters: multiply length by width by thickness, divide by 27 for cubic yards, then add about 10 percent for waste.Read guideHome & GardenHow to calculate square footageCalculate square footage (or square meters) by multiplying length by width. Convert inches to decimal feet, split L-shaped rooms into rectangles, and add the areas.Read guideHome & GardenHow much mulch do I need?Mulch depth divided by 12, times bed area, gives cubic feet. Divide by 27 for cubic yards, or work in meters for cubic meters, or by 2 for standard bags. Worked example inside.Read guideHome & GardenHow much flooring do I need?Work out how much flooring to buy in square feet or square meters: room area times one plus a waste fraction, divided by box coverage, rounded up. Free calculator and worked example.Read guideFinanceHow to convert hourly pay to an annual salaryConvert hourly pay to a yearly salary with one simple formula, full-time and part-time examples, and how gross pay differs from your take-home.Read guideBusinessHow to calculate profit marginCalculate profit margin: subtract cost from revenue for profit, then divide profit by revenue. Includes a worked example and how margin differs from markup.Read guideAutoHow much will my road trip cost?Road trip cost = fuel plus lodging, food, tolls, parking, and activities. See the formula, a worked 1,200-mile (1,900 km) example, and why gas is rarely the biggest cost.Read guidePlanning ToolsMoving checklist: what to do before moving dayA moving checklist counted back from moving day: research and declutter at 8 weeks, confirm the mover and transfer utilities at 2 weeks, pack last.Read guideFinanceHow to calculate your monthly mortgage paymentCalculate a monthly mortgage payment with the standard amortization formula, a worked $320,000 loan at 6.5% over 30 years (about $2,023 principal and interest), plus how taxes and insurance build PITI.Read guideHealthHow to calculate your TDEECalculate your TDEE with the Mifflin-St Jeor BMR equation and an activity factor. A worked example lands at about 2,731 calories, plus deficit, surplus, and macro targets. Estimates, not medical advice.Read guideFinanceHow compound interest works (and why starting early matters)How compound interest works: the A = P(1 + r/n)^(nt) formula, the Rule of 72, and a worked $10,000 at 7% example growing to about $81,164 in 30 years. Estimates only.Read guideHealthHow to calculate and understand your BMICalculate your BMI in metric or US units with a worked example (5'9", 165 lb, BMI 24.4), the standard weight categories, and an honest look at what BMI misses. A screening tool, not a diagnosis.Read guideEveryday ToolsHow to calculate your GPA (weighted and unweighted)Calculate your GPA on the 4.0 scale: grade points, credit-hour weighting, and the difference between weighted and unweighted GPA. A worked 5-course example landing at 3.13 unweighted and 3.53 weighted.Read guideFinanceHow does federal paycheck tax withholding work?How federal paycheck withholding works: gross vs. net pay, the W-4, progressive income tax after the standard deduction, FICA, and why you get a refund or owe. Worked $60,000 example. Estimates only.Read guideFinanceHow amortization works (and how to read your amortization schedule)How amortization works and how to read your loan schedule: the payment formula, a worked $25,000 example at 5% APR for 5 years, and how extra principal payments save interest.Read guideEveryday ToolsHow to calculate percentagesHow to calculate percentages: find X percent of Y, find what percent X is of Y, and find percent increase or decrease, each with a worked example and the formula.Read guideAutoHow to calculate your car loan paymentCalculate a car loan payment step by step using the standard amortization formula. Worked example: $25,000 financed at 6.9% APR over 60 months, plus trade-in, tax, and term trade-offs.Read guideFinanceHow much house can you afford?Figure out how much house you can afford using the 28/36 debt-to-income rule. Worked example: $8,000/month income, $500 in debt, and a real loan-amount estimate at 6.5%.Read guideFinanceHow do you calculate your debt-to-income ratio (DTI)?Calculate your debt-to-income ratio with the DTI formula, a worked $6,500-income example at 38.5%, what counts as a good DTI, and how to lower it.Read guideHealthHow do you calculate your one-rep max (1RM)?Estimate your one-rep max without testing it: the Epley and Brzycki formulas, a worked 185 lb bench press example, and a training percentage table.Read guideConstructionHow much fencing do you need?Work out how much fencing you need: perimeter, post spacing, and a worked 320-ft example (100x60 ft yard) with exact post and section counts. Estimates only.Read guideConstructionHow many roofing shingles do you need?Work out how many roofing shingles you need: roof squares from footprint and pitch, a worked 1,800 sq ft / 6-12 pitch example, waste factor, and bundle counts.Read guideHealthHow much water should you drink a day?A rule-of-thumb water target (half an ounce per pound, plus exercise and heat) with worked examples, and how it compares with National Academies intake figures. Estimates, not medical advice.Read guideHealthHow to calculate a calorie deficit for your goalCalculate the daily calorie deficit for your weight-loss goal using the 3,500 kcal per lb (7,700 per kg) rule, with worked examples and honest limits. A planning estimate, not medical advice.Read guideFinanceHow to use the 50/30/20 budget ruleUse the 50/30/20 budget rule step by step: split take-home pay into needs, wants, and savings, with a worked $4,800 example and what to do when needs top 50 percent. Not financial advice.Read guideBusinessMarkup vs. margin: what the difference is and how to convert between themMarkup divides profit by cost; margin divides it by price. Learn both conversion formulas, see worked examples, and avoid the pricing mistake that quietly cuts your profit.Read guideBusinessHow to calculate your break-even pointCalculate your break-even point in units and dollars: fixed costs divided by contribution margin per unit. Includes a worked example, profit targets, and common mistakes.Read guideFinanceHow to calculate sales tax and discounts togetherAdd sales tax, back it out of a receipt total, stack discounts correctly (25% then 10% is 32.5%, not 35%), and know whether discounts come before tax. Worked examples included.Read guideAutoHow to calculate your MPG and fuel costCalculate your real MPG with the fill-to-fill method, convert to L/100 km, avoid the MPG illusion, and see why real-world mileage differs from the EPA label. Worked examples included.Read guideAutoHow to read and compare tire sizesDecode 225/60R16, calculate overall tire diameter, compare two sizes, and see the speedometer error. Worked plus-sizing examples with real numbers, plus load index and speed rating basics.Read guideAutoHow much does it cost to charge an electric car?Work out what it costs to charge an electric car: energy added, charging losses, and your price per kWh. Worked example for a 75 kWh battery, cost per mile, and home vs public charging.Read guideHome & GardenHow much gravel do I need?Work out how much gravel to order: area times depth gives cubic yards, then about 1.4 tons per cubic yard gives weight. Depth by use, compaction, and tonnes in metric.Read guideConstructionHow many bricks do I need for a wall?Estimate bricks for a wall: wall area times about 7 bricks per square foot (60 per square meter), plus waste. Learn to subtract openings and handle double-wythe walls.Read guideUnit ConvertersHow to convert Celsius, Fahrenheit, and KelvinConvert between Celsius, Fahrenheit, and kelvin with the exact formulas: F = C x 9/5 + 32, K = C + 273.15. Includes the -40 crossover, oven and weather examples, and absolute zero.Read guideFinanceHow to calculate a tip and split the billCalculate a tip, decide between pre-tax and post-tax, handle rounding, and split a bill evenly or by what each person ordered, with worked examples.Read guideFinanceHow much should you save, and how do savings grow?How monthly savings compound, how APY differs from APR, and why inflation matters, with worked examples and an emergency fund plan.Read guideBusinessHow to set your freelance hourly rateSet a freelance hourly rate from the income you need, your expenses, and realistic billable hours, plus how self-employment tax changes it.Read guideConstructionHow to calculate stair risers and treadsDivide total rise by a target riser height, round to a whole number, then get treads (risers minus 1) and total run. Worked inch and cm examples.Read guideConstructionHow much insulation do I need?Find the area, add 5 percent waste, divide by the coverage on the package, and round up. Plus R-value basics and U.S. DOE climate-zone targets.Read guideHome & GardenHow many deck boards do I need?Count rows across the deck width, convert to linear feet, divide by board length, add waste. Worked 16 by 12 ft example, gaps, and stagger.Read guideHealthHow to calculate your body fat percentage with a tape measureEstimate body fat with the U.S. Navy tape-measure method: how to measure, worked examples, and honest accuracy limits compared with DEXA.Read guideHealthHow your pregnancy due date is calculatedHow a due date is calculated: Naegele's rule (280 days from your last period), the 266-day conception method, and ultrasound dating.Read guideHome & GardenHow many tiles do I need?Work out how many tiles to buy: area x (1 + waste) divided by tile size, with worked examples, grout joint effects, layout waste, and spare tiles.Read guideHome & GardenHow much sod or grass seed do I need?Calculate sod rolls and pallets or pounds of grass seed for a new lawn, with worked examples, extra-order allowances, sod vs seed tradeoffs, and timing.Read guideHome & GardenHow much soil do I need for a raised bed?Calculate the soil for a raised bed in cubic feet, cubic yards, and bags, with depth, soil and compost mix, settling, and bulk versus bagged tradeoffs.Read guideHealthHow ideal weight formulas work (and why they are not targets)How the Devine, Robinson, Miller and Hamwi ideal weight formulas work, where they came from, why they disagree, and why none is a target.Read guideEveryday ToolsHow to create a strong password: length, entropy, and what actually mattersWhat makes a password strong: entropy math (length x log2 of the character set), why length beats symbols, passphrases, password managers, and MFA.Read guideEveryday ToolsHow to calculate age and the days between two datesCalculate exact age in years, months and days, plus days between dates. Covers calendar borrowing, leap years, inclusive counting, month lengths.Read guideUnit ConvertersHow to convert between metric and imperial unitsConvert metric and imperial units with exact definitions (1 in = 25.4 mm, 1 lb = 0.45359237 kg), squared and cubed factors, and common mistakes.Read guide