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 guide