# Howdeedo Premium calculator website and deterministic computation platform. Fast, trustworthy calculators with clear explanations for humans. Versioned APIs with structured, citation-ready responses for AI agents and developers. ## Available calculators - mortgage: Monthly payment, total interest, and what happens when you change the rate by half a point. (Spoiler: it's a lot.) - 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. - bmi: Height, weight, one number. We'll tell you what the ranges mean and what they don't. - unit-convert: Cups to liters, Fahrenheit to Celsius, pounds to kilograms. The stuff you Google three times a week. - percentage: What's 18% of $47.50? Percent change? Increase, decrease? We won't judge why you need this at 11pm. - tip: Tip, total, and split — so the table isn't doing mental math while the server waits. - compound-interest: How $10K becomes $30K (or $100K) depending on rate, time, and whether you keep adding to it. - sales-tax: Add or remove sales tax. Pre-tax to total or total to pre-tax. - age: Age in years, months, and days from birth date. Total days. - date-diff: Days, weeks, and months between two dates. - test-grade: Score and total to percentage and letter grade. Standard scale. - salary-hourly: Convert between annual salary and hourly rate. Monthly and weekly. - gpa: Letter grades and credits to cumulative GPA. Standard 4.0 scale. - calorie: BMR and TDEE from weight, height, age, sex, and activity. Mifflin-St Jeor. - retirement: Projected balance from starting amount, annual contribution, rate, and years. - pregnancy-due-date: Estimated due date from LMP. Naegele rule. For planning only. - child-growth-bmi: BMI-for-age percentile for children 0-19 years. WHO growth standards, sex-specific. - random-number: Generate a random integer between min and max. Cryptographically secure. - time-duration: Duration between start and end times. Handles day boundaries automatically. - discount: Final price and savings from percentage discount. Perfect for sales and coupons. - simple-interest: Interest and total from principal, rate, and time. Linear interest, no compounding. - pace: Minutes per mile or km from distance and time. Perfect for runners and race planning. - child-growth-wfl: WHO weight percentiles for infants 0-24 months. Weight-for-length, sex-specific. - child-growth: WHO growth percentiles with multiple measurements. Height-for-age, weight-for-age, and more. - p-value: Statistical significance from z-test or t-test. Hypothesis testing at α = 0.05. - tax-bracket: US Federal income tax for 2024. Total tax, effective rate, marginal rate by bracket. - debt-payoff: Months to payoff and total interest from balance, rate, and payment. Credit card planning. - currency-convert: Convert between major currencies. Fixed rates, quick estimates for travel and shopping. - fuel-cost: Trip fuel cost from distance, MPG/L100km, and price. Road trip budgeting. - bac: Blood Alcohol Content estimate from drinks, weight, sex, and time. Widmark formula. ESTIMATE ONLY. - paint-estimator: Estimate paint quantity for walls and ceilings with coats, coverage rate, and buy rounding. - mulch-estimator: Estimate mulch or soil volume from area and depth, with cubic yards and bag count outputs. - gravel-estimator: Estimate gravel quantity by area and depth with cubic yards, tons, and density-aware conversion. - concrete-slab-estimator: Estimate concrete slab volume from length, width, and thickness with cubic yard and bag-style outputs. - drywall-estimator: Estimate drywall sheet count for walls and ceilings with waste adjustment and rounded purchase totals. - flooring-estimator: Calculate flooring boxes for wood, laminate, or LVP. Boxes needed with waste factor. - tile-estimator: Calculate tile count for floors and walls. Accounts for tile size and waste. - trim-estimator: Calculate baseboard and trim sticks. Linear feet to pieces needed. - fertilizer-estimator: Calculate lawn fertilizer bags. Based on nitrogen content and application rate. - seed-sod-estimator: Calculate grass seed or sod rolls. Choose seed or sod mode for lawn coverage. - materials-estimator: Material estimators for paint, flooring, tile, concrete, and more. Pick a calculator from the hub. ## URLs - Home: https://www.howdeedo.com/ - OpenAPI: https://www.howdeedo.com/api/spec - Health: https://www.howdeedo.com/api/health - Mortgage: https://www.howdeedo.com/mortgage | https://www.howdeedo.com/api/calc/mortgage - House affordability: https://www.howdeedo.com/house-affordability | https://www.howdeedo.com/api/calc/house-affordability - Loan: https://www.howdeedo.com/loan | https://www.howdeedo.com/api/calc/loan - BMI: https://www.howdeedo.com/bmi | https://www.howdeedo.com/api/calc/bmi - Unit converter: https://www.howdeedo.com/unit-converter | https://www.howdeedo.com/api/calc/unit-convert - Percentage: https://www.howdeedo.com/percentage | https://www.howdeedo.com/api/calc/percentage - Tip: https://www.howdeedo.com/tip | https://www.howdeedo.com/api/calc/tip - Compound interest: https://www.howdeedo.com/compound-interest | https://www.howdeedo.com/api/calc/compound-interest - Sales tax: https://www.howdeedo.com/sales-tax | https://www.howdeedo.com/api/calc/sales-tax - Age: https://www.howdeedo.com/age | https://www.howdeedo.com/api/calc/age - Date difference: https://www.howdeedo.com/date-diff | https://www.howdeedo.com/api/calc/date-diff - Test grade: https://www.howdeedo.com/test-grade | https://www.howdeedo.com/api/calc/test-grade - Salary to hourly: https://www.howdeedo.com/salary-hourly | https://www.howdeedo.com/api/calc/salary-hourly - GPA: https://www.howdeedo.com/gpa | https://www.howdeedo.com/api/calc/gpa - Calorie: https://www.howdeedo.com/calorie | https://www.howdeedo.com/api/calc/calorie - Retirement: https://www.howdeedo.com/retirement | https://www.howdeedo.com/api/calc/retirement - Pregnancy due date: https://www.howdeedo.com/pregnancy-due-date | https://www.howdeedo.com/api/calc/pregnancy-due-date - Child growth BMI: https://www.howdeedo.com/child-growth-bmi | https://www.howdeedo.com/api/calc/child-growth-bmi - Random number generator: https://www.howdeedo.com/random-number | https://www.howdeedo.com/api/calc/random-number - Time duration: https://www.howdeedo.com/time-duration | https://www.howdeedo.com/api/calc/time-duration - Discount: https://www.howdeedo.com/discount | https://www.howdeedo.com/api/calc/discount - Simple interest: https://www.howdeedo.com/simple-interest | https://www.howdeedo.com/api/calc/simple-interest - Running pace: https://www.howdeedo.com/pace | https://www.howdeedo.com/api/calc/pace - Child growth weight-for-length: https://www.howdeedo.com/child-growth-wfl | https://www.howdeedo.com/api/calc/child-growth-wfl - Child growth chart: https://www.howdeedo.com/child-growth | https://www.howdeedo.com/api/calc/child-growth - P-value: https://www.howdeedo.com/p-value | https://www.howdeedo.com/api/calc/p-value - Tax bracket: https://www.howdeedo.com/tax-bracket | https://www.howdeedo.com/api/calc/tax-bracket - Debt payoff: https://www.howdeedo.com/debt-payoff | https://www.howdeedo.com/api/calc/debt-payoff - Currency converter: https://www.howdeedo.com/currency-convert | https://www.howdeedo.com/api/calc/currency-convert - Fuel cost: https://www.howdeedo.com/fuel-cost | https://www.howdeedo.com/api/calc/fuel-cost - Blood alcohol content: https://www.howdeedo.com/bac | https://www.howdeedo.com/api/calc/bac - Paint estimator: https://www.howdeedo.com/paint-estimator | https://www.howdeedo.com/api/calc/paint-estimator - Mulch estimator: https://www.howdeedo.com/mulch-estimator | https://www.howdeedo.com/api/calc/mulch-estimator - Gravel estimator: https://www.howdeedo.com/gravel-estimator | https://www.howdeedo.com/api/calc/gravel-estimator - Concrete slab estimator: https://www.howdeedo.com/concrete-slab-estimator | https://www.howdeedo.com/api/calc/concrete-slab-estimator - Drywall estimator: https://www.howdeedo.com/drywall-estimator | https://www.howdeedo.com/api/calc/drywall-estimator - Flooring estimator: https://www.howdeedo.com/flooring-estimator | https://www.howdeedo.com/api/calc/flooring-estimator - Tile estimator: https://www.howdeedo.com/tile-estimator | https://www.howdeedo.com/api/calc/tile-estimator - Trim estimator: https://www.howdeedo.com/trim-estimator | https://www.howdeedo.com/api/calc/trim-estimator - Fertilizer estimator: https://www.howdeedo.com/fertilizer-estimator | https://www.howdeedo.com/api/calc/fertilizer-estimator - Seed & sod estimator: https://www.howdeedo.com/seed-sod-estimator | https://www.howdeedo.com/api/calc/seed-sod-estimator - Materials estimator: https://www.howdeedo.com/materials-estimator ## For agents - Example (mortgage): GET https://www.howdeedo.com/api/calc/mortgage?homePrice=350000&downPayment=70000&downPaymentMode=amount&annualRatePct=6.5&termYears=30 - API docs (per calculator): https://www.howdeedo.com/api/docs/{slug} (e.g. https://www.howdeedo.com/api/docs/mortgage)