UK DIY Cost Calculator
Concrete Calculator (Bags & Cost)
Work out the volume of concrete you need and how many bags of ready-mix or postmix that means, plus the cost. Enter the dimensions of your slab, footing or post hole.
Your project
Estimate
Estimates are for guidance only. Coverage rates, pack sizes and prices vary by brand — always confirm quantities with your supplier before buying.
From dimensions to bags
Concrete is calculated by volume: length times width times depth, all in metres. The trick most people miss is the depth — a 100mm slab is 0.1m, not 1. Once you have the volume, dividing by a bag's yield gives the number of bags. A typical 20kg bag of postmix or ready-mix yields around 0.011 m³, but check yours, as it varies by product.
When to order a mixer load instead
Bags are convenient for post holes, small footings and repairs, but they get expensive and laborious fast. Once you're past roughly half a cubic metre, ready-mixed delivered by the barrow-load or a mix-on-site lorry is usually cheaper and far less work than mixing dozens of bags by hand.
Frequently asked questions
How many bags of concrete per cubic metre?
At a typical yield of about 0.011 m³ per 20kg bag, you need roughly 90 bags per cubic metre. That's why larger pours are usually cheaper ordered as ready-mix.
How do I calculate concrete for a slab?
Multiply length x width x thickness in metres to get the volume, then add about 10% for waste and uneven ground. A 3m x 2m slab at 100mm is 0.6 m³ before waste.
Should I use bags or ready-mix?
Bags suit small jobs like fence posts and repairs. For anything over about half a cubic metre, ready-mixed or mix-on-site is generally more economical and much quicker than hand-mixing.
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