How we work it out

Last updated 3 Oct 2026

This page explains how our calculators get their answers. Every calculator also shows its own steps, with your numbers.

1. We start with your sizes

You type the length, width and depth. The calculator turns them into a volume, like cubic feet or cubic metres. The maths runs on your phone or computer. We don't save your sizes. They show in the page address, so you can share your answer.

2. We add a little extra

Some material is always lost to spills and uneven ground. You choose how much extra to add. For concrete, Sakrete, a bag maker, suggests about 10%. For stone, sand and soil, one supplier says to add 10%.

3. We allow for packing down

Gravel and stone shrink when you pack them down. You type the depth after packing. We work out how much loose stone to order. We never count the packing twice.

4. We round up what you buy

• Bags, pails and rolls are always rounded up. You can't buy part of a bag.
• Truck orders round up to the next quarter yard (or quarter of a cubic metre). This is our own rule. Ask your supplier for theirs.
• We round numbers only to show them. The maths itself is not rounded.

5. Where the numbers come from

• How much a bag makes: the maker's data sheet or bag label.
• How heavy a material is: suppliers, quarries and government road guides.
• Prices: we collect prices from stores, suppliers, price lists and cost guides. We show the middle price, how many we found, and the date we checked. You can change any price.
• How thick or deep: building codes and guides from makers and towns.

Each page lists its sources at the bottom.

6. We keep it up to date

• Every month we check that our source links still work.
• We update prices regularly, and every January, when many suppliers change them.
• Each page shows the date we last checked it.

7. What our answers are

Our answers are a guide, based on your sizes. Real jobs vary. For footings, driveways or anything that holds weight, check your local building rules or ask a builder or engineer. Always check prices with your store.

Found a mistake? Please tell us.