All calculators

Every tool on Howdeedo, grouped by category. Same deterministic math on the web and in the API. Questions? See the FAQ or contact us. For a flat directory aimed at discovery and crawlers, see the site map.

Finance

  • Compound interest — How $10K becomes $30K (or $100K) depending on rate, time, and whether you keep adding to it.
  • Debt payoff — Months to payoff and total interest from balance, rate, and payment. Credit card planning.
  • Discount — Final price and savings from percentage discount. Perfect for sales and coupons.
  • House affordability — Total monthly housing cost: rent or mortgage, taxes, insurance, maintenance, and more. See what you'll pay and what share of income it is.
  • Loan — Monthly payment and total interest for a fixed-rate loan. Personal loans, auto loans—same formula lenders use.
  • Mortgage — Monthly payment, total interest, and what happens when you change the rate by half a point. (Spoiler: it's a lot.)
  • Retirement — Projected balance from starting amount, annual contribution, rate, and years.
  • Salary to hourly — Convert between annual salary and hourly rate. Monthly and weekly.
  • Sales tax — Add or remove sales tax. Pre-tax to total or total to pre-tax.
  • Simple interest — Interest and total from principal, rate, and time. Linear interest, no compounding.
  • Tax bracket — US Federal income tax for 2024. Total tax, effective rate, marginal rate by bracket.

Health & fitness

  • Blood alcohol content — Blood Alcohol Content estimate from drinks, weight, sex, and time. Widmark formula. ESTIMATE ONLY.
  • BMI — Height, weight, one number. We'll tell you what the ranges mean and what they don't.
  • Calorie — BMR and TDEE from weight, height, age, sex, and activity. Mifflin-St Jeor.
  • Child growth BMI — BMI-for-age percentile for children 0-19 years. WHO growth standards, sex-specific.
  • Child growth chart — WHO growth percentiles with multiple measurements. Height-for-age, weight-for-age, and more.
  • Child growth weight-for-length — WHO weight percentiles for infants 0-24 months. Weight-for-length, sex-specific.
  • Pregnancy due date — Estimated due date from LMP. Naegele rule. For planning only.
  • Running pace — Minutes per mile or km from distance and time. Perfect for runners and race planning.

Math & education

  • GPA — Letter grades and credits to cumulative GPA. Standard 4.0 scale.
  • P-value — Statistical significance from z-test or t-test. Hypothesis testing at α = 0.05.
  • Percentage — What's 18% of $47.50? Percent change? Increase, decrease? We won't judge why you need this at 11pm.
  • Random number generator — Generate a random integer between min and max. Cryptographically secure.
  • Test grade — Score and total to percentage and letter grade. Standard scale.

Everyday

  • Concrete slab estimator — Estimate concrete slab volume from length, width, and thickness with cubic yard and bag-style outputs.
  • Currency converter — Convert between major currencies. Fixed rates, quick estimates for travel and shopping.
  • Drywall estimator — Estimate drywall sheet count for walls and ceilings with waste adjustment and rounded purchase totals.
  • Fertilizer estimator — Calculate lawn fertilizer bags. Based on nitrogen content and application rate.
  • Flooring estimator — Calculate flooring boxes for wood, laminate, or LVP. Boxes needed with waste factor.
  • Fuel cost — Trip fuel cost from distance, MPG/L100km, and price. Road trip budgeting.
  • Gravel estimator — Estimate gravel quantity by area and depth with cubic yards, tons, and density-aware conversion.
  • Materials estimator — Material estimators for paint, flooring, tile, concrete, and more. Pick a calculator from the hub.
  • Mulch estimator — Estimate mulch or soil volume from area and depth, with cubic yards and bag count outputs.
  • Paint estimator — Estimate paint quantity for walls and ceilings with coats, coverage rate, and buy rounding.
  • Seed & sod estimator — Calculate grass seed or sod rolls. Choose seed or sod mode for lawn coverage.
  • Tile estimator — Calculate tile count for floors and walls. Accounts for tile size and waste.
  • Tip — Tip, total, and split — so the table isn't doing mental math while the server waits.
  • Trim estimator — Calculate baseboard and trim sticks. Linear feet to pieces needed.
  • Unit converter — Cups to liters, Fahrenheit to Celsius, pounds to kilograms. The stuff you Google three times a week.

Date & time

  • Age — Age in years, months, and days from birth date. Total days.
  • Date difference — Days, weeks, and months between two dates.
  • Time duration — Duration between start and end times. Handles day boundaries automatically.

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