How much do you actually need?

Almost every wrong answer starts with a wrong measurement rather than wrong arithmetic. Four things are worth getting right before you type anything in.

• Measure the inside of a bed, not the outside. A bed built from two-inch lumber is four inches narrower inside than its outside dimensions suggest. On a four-by-eight bed that is most of a bag of soil.
• Decide the fill depth, not the wall height. Soil is normally filled to an inch or two below the rim so watering does not wash it over the edge. Use the depth you will actually fill to.
• Know whether the bed has a bottom. A bed sitting on open soil can be part-filled with coarse material or native soil underneath; a bed on a patio or with a solid base needs the whole volume in bought mix.
• Check the bag, not the brand. Bagged soil and compost come in one, one and a half, two and three cubic foot bags, and mulch bags differ again. The bag size is the number that decides how many bags you carry home.

Expect to buy a little more than any calculator says. Fresh mixes settle as the organic matter in them breaks down, so a bed filled level in April usually wants topping up by the following spring. Ten per cent extra is a reasonable allowance for a raised bed, and for mulch, which compacts as it weathers.

All twelve tools behave the same way, because the point is to get an answer and move on.

• The result updates as you type. There is no submit button. If a value does not make sense — a depth of zero, a negative length — the tool says so next to the field instead of showing you a broken number.
• Imperial or metric, your choice. Every tool has a unit toggle and remembers which you picked, so it opens the way you left it next time.
• The answer comes as a sentence. Not just “32”, but how many bags that is, and what it comes to in cubic yards or liters, so you can check it against what is stacked on the pallet.
• Your numbers live in the page address. Copy the link and the result travels with it — useful when one person is measuring and another is at the shop.
• The formula is on the page. Each tool shows the arithmetic it used with a worked example, so nothing is a black box and you can redo it by hand.

Nothing is sent anywhere. The calculators run entirely in your browser, there is no account, and the only thing stored on your device is your unit preference and any garden plan you choose to save.