How the gravel calculator works

Area of each shape

• Rectangle: length × width.
• L-shape: overall length × overall width, minus notch length × notch width. The notch must be smaller than the outline in both directions.
• Circle: π × (diameter ÷ 2)².
• Triangle: base × height ÷ 2, where height is measured at a right angle to the base.
• Trapezoid: (long side + short side) ÷ 2 × the distance between those two parallel sides.
• Known area: used as entered.

Volume and the extra allowance

Each area's volume is its surface area × its depth. The volumes are added to give the measured total. Compaction and waste are both percentages of that measured total and are added together: 10% compaction and 5% waste means measured total × 1.15. They are not compounded.

Weight

Weight = volume × density. You can type your supplier's density, or pick a material to fill in a typical value. Each material has a typical range of loose density (for example pea gravel 95-110 lb/ft³, crusher run 105-130 lb/ft³, topsoil 70-100 lb/ft³). These are approximate ranges of the kind found in supplier conversion charts, not measurements, so when a material is picked the result shows tons as a range across it. Your supplier's figure for the exact product is always more accurate.

Unit definitions

• 1 foot = 0.3048 metres exactly, so 1 cubic yard = 27 cubic feet = 0.764555 cubic metres.
• 1 pound = 0.45359237 kilograms exactly.
• 1 US (short) ton = 2,000 pounds. 1 metric tonne = 1,000 kilograms.
• 1 US gallon = 231 cubic inches, so a 5 gallon bucket holds 0.668 cubic feet.

These are the definitions published by the US National Institute of Standards and Technology (NIST).

Rounding

• Bags, wheelbarrow trips and truck loads are always rounded up to a whole number, because you cannot buy part of a bag.
• Before rounding up, results are trimmed to 12 significant digits. Computers store some decimals inexactly; 55 cubic feet can come out as 55.00000000000001, which would otherwise round up to an extra bag.
• Volumes and weights are shown to two decimal places. Nothing else is rounded until the final display.

Bags, trips and loads

• Bag counts are shown for 0.5 ft³ and 1 ft³ bags, and for 50 lb bags once a density is entered.
• A wheelbarrow trip is taken as 3 cubic feet. A 6 cubic foot contractor barrow is usually filled about halfway with stone, because a full load is too heavy to push safely.
• Truck loads use the load size you enter, because it depends on the supplier's trucks and local weight limits.

French drains

Gravel = trench length × width × gravel depth, minus the pipe: π × (pipe diameter ÷ 2)² × length. The pipe is assumed to run the full length of the trench and sit entirely inside the gravel.

Coverage

Area covered = volume ÷ depth. Depth reached = volume ÷ area. When the amount is given as a weight, it is first turned into volume using the density you enter.

Testing

The calculation code has automated tests for every shape, the allowance math, unit round trips, coverage in both directions, bag and load counts, the French drain pipe subtraction and the floating-point rounding case above. The tests run before every change is published.