How we calculate
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This page explains how every planner on this site turns your measurements into a shopping list. Nothing here is hidden: each default is visible on the planner page, and you can change it.
Areas
Each project is split into simple shapes. The planners use these standard formulas:
- Rectangle: length × width
- Square: side × side
- Triangle: ½ × base × height
- Circle: π × (diameter ÷ 2)²
- Half circle: ½ × π × (diameter ÷ 2)²
- Trapezoid: ½ × (side A + side B) × height
- Curved border: length along the curve × average width
- Known area: the area you enter
Every area can be added or subtracted. The net area is the sum of the added areas minus the subtracted ones. If subtracted areas are as large as the added ones, the planner stops and asks you to check them instead of showing a zero or negative result.
Volumes and weights
Volume is area × depth. The amount to order is the net volume plus the allowances shown in the Assumptions panel, for example net volume × (1 + compaction + waste) for gravel. Weight is volume × density. Concrete for a fence post is the hole’s cross-section minus the post’s, times the height of concrete in the hole.
Units and conversions
Every calculation runs in metric units (meters, cubic meters and kilograms). Imperial values are converted on the way in and converted back only for display, using exact factors:
- 1 ft = 0.3048 m, and 1 in = 0.0254 m
- 1 yd³ = 0.764554858 m³ (27 ft³)
- 1 lb = 0.45359237 kg
- 1 US ton = 2,000 lb = 907.18474 kg; 1 tonne = 1,000 kg
When you switch between Imperial and Metric, the values you typed are converted exactly. Defaults you have not changed switch to the round default for the other system, for example 3 in becomes 75 mm, so the two modes can give slightly different answers. You can type decimals with a point or a comma, and fractions such as 2 3/8.
Rounding
Anything you buy in whole units, such as bags, pavers, posts, edging pieces and delivery loads, is always rounded up. To avoid floating-point surprises like 2.0000000001 turning into 3, the planners round up with a tiny tolerance. Bulk amounts are rounded up to the step you choose, such as half a cubic yard.
Why densities, bag sizes and allowances are editable
Real materials vary. Gravel from one quarry is heavier than from another; wet sand weighs more than dry sand; bags of the same weight can yield different volumes; mulch settles more than gravel. Each default is a reasonable starting point, not a recommendation, and is shown next to the field so you can compare it with your product label or your supplier’s figures. Changed values are highlighted and printed on your shopping list.
What the planners do not do
The planners estimate quantities only. They do not recommend depths, base designs, post depths, spacing, frost depth or drainage, and they do not check building codes, permits or structural safety. They are not a substitute for the product manufacturer’s instructions or professional advice.
How the formulas are checked
Each planner has worked examples, calculated by hand, that are also part of an automated test suite. The tests check areas, volumes, weights and bag counts against the expected values, check that Imperial and Metric give consistent results, and check that missing, zero and negative values are blocked. The same worked examples appear on each planner page, and you can load them with the “Try the worked example” button.
Changelog
- : First public version of all five planners.
Found a calculation error?
If a result looks wrong, please contact us with the planner name, the values you entered and what you expected. Corrections are welcome.