How do I calculate how much concrete I need?

Multiply length by width by thickness (converted to the same unit) to get volume, then add a waste margin (typically 5-10%) and divide by the yield of the bag you're using to get the number of bags.

Why is bag yield editable instead of fixed?

Bag yield varies by brand, bag size, and product type. The yield is printed on the bag's packaging — enter that exact number for an accurate bag count.

Concrete Calculator — Volume & Bags Needed

How concrete volume is calculated

Volume for a slab or footing is simply length times width times thickness, once all three are converted to matching units.

volume = length × width × thickness

In metric, length and width are in meters and thickness in centimeters (divided by 100 to convert to meters), giving cubic meters. In imperial, length and width are in feet and thickness in inches (divided by 12), giving cubic feet.

Why add a waste margin

Uneven subgrade, spillage, and small measurement errors mean the exact calculated volume is rarely enough on-site. A 5-10% waste margin is standard practice for most pours; larger or more irregular jobs may need more.

About bag yield

This calculator is built around a standard 50 kg cement bag, combined on-site with sand, gravel and water — not a pre-mixed ready-to-pour bag, which is a different product with a much lower yield. As a rule of thumb:

Floor / slab concrete (cement + sand + gravel): one 50 kg bag typically produces enough mix to cover roughly 2–2.4 m² at a 10 cm thickness. In terms of fresh mix volume, that's commonly cited as around 0.075–0.08 m³ per bag — the calculator defaults to 0.078 m³.
Wall render / plaster (cement + sand only, no gravel): one bag stretches much further in a thin layer — around 8 m² at 1.5 cm thickness, which works out to roughly 0.12 m³ per bag.

Worth being upfront about: rule-of-thumb yield figures circulating online are inconsistent with each other — the coverage-based numbers (m² at a given thickness) and the volume-based numbers (m³ of fresh mix) don't always agree, because they come from different measurement methods (loose mix volume versus set-in-place coverage). For anything beyond a small DIY job, confirm the mix ratio and yield with your local supplier or contractor.

Common uses

Slabs and patios: estimating material for a backyard pour.
Footings and posts: sizing small concentrated pours for fence posts or footings.
Budgeting: figuring out how many bags to buy before a trip to the hardware store.

Frequently asked questions

Does this work for irregular shapes?

This calculator assumes a rectangular pour. For an irregular area, split it into rectangular sections, calculate each separately, and add the volumes together.

Should I use ready-mix delivery instead of bags for a large pour?

For volumes above roughly 1 m³ (about 1.3 cubic yards), ready-mix truck delivery is usually more practical and often cheaper per unit than bagged concrete — this calculator's bag count is most useful for smaller DIY jobs.