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Calculation methodology
The formulas, constants and rounding rules behind the tools that are live today.
General principles
- Full precision internally; rounding happens only for display (up to 6 significant digits for conversions, cents for money).
- Every value is shown with its unit.
- Inputs are validated; impossible values show an error instead of a result.
- Calculations run in your browser. Results do not depend on a server.
Unit conversions
Each unit is converted through an SI base unit using exact defined factors:
| Unit | Definition |
|---|---|
| inch | 0.0254 m (exact) |
| foot | 0.3048 m (exact) |
| mile | 1,609.344 m (exact) |
| pound (avoirdupois) | 0.45359237 kg (exact) |
| ounce | 1/16 lb = 28.349523125 g |
| US gallon | 3.785411784 L (exact) |
| US fluid ounce | 1/128 US gal ≈ 29.5735 mL |
| US cup | 8 US fl oz ≈ 236.5882 mL |
| acre | 4,046.8564224 m² (43,560 ft²) |
| psi | 6,894.757293168 Pa |
| pound-foot | 1.3558179483 N·m |
| average month | 30.436875 days (365.2425 ÷ 12) |
Temperature: °F = °C × 9/5 + 32; K = °C + 273.15.
Date arithmetic
Dates are calculated in UTC calendar days so daylight-saving changes never shift a result. Adding months keeps the day of the month, or uses the month’s last day when that day doesn’t exist. Business days skip Saturdays and Sundays; public holidays are not yet excluded. Week numbers follow ISO 8601.
Concrete volume
Bag counts use typical printed yields of US pre-mix bags (40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³) and always round up. Weight uses 150 lb/ft³, a common planning value for normal-weight concrete.
Loan amortization
Interest is computed monthly on the remaining balance and rounded to the cent. The final payment is adjusted so the balance reaches exactly $0. A 0% rate gives M = P ÷ n.
Testing
Live formulas have automated tests with known answers (nearly 500 checks run before every release), for example 90 days after 30 Oct 2026 is 28 Jan 2027, a 10 × 10 ft × 4 in slab is 1.2346 yd³, and 1 acre is 43,560 ft².