Concrete Calculator (Bags & Cost)

How to budget for concrete

Concrete quantities are governed by volume rather than surface area. Convert every measurement to metres before multiplying length by width by depth, then choose bagged concrete, ready-mix or a site-mixed specification that suits the pour.

Use the calculator for a quantity-led starting point, then check it against the chosen manufacturer's technical information and a supplier quote. Product dimensions, coverage and pack sizes change, and a low advertised unit rate can be offset by delivery, minimum orders or accessories. Keep the measurements, quantity calculation and cost assumptions together so they can be updated without remeasuring the project.

Typical UK concrete prices

The figures below are illustrative UK ranges as at September 2026, including VAT where normally charged. Supply-only means the main named material and does not automatically include preparation, delivery, accessories or waste. Supplied-and-fitted ranges are broad consumer budgeting figures, not quotations; London, difficult access and unusually small jobs can cost more.

Prices are shown per m³ or item as appropriate. Compare like with like: clarify VAT, delivery, removal of old material, preparation, finishing and guarantees. A contractor rate may include several of these, while a retailer's headline price generally does not. Obtain current local prices before ordering or agreeing work.

Worked example: calculation step by step

Take a 4m by 3m garden shed base poured 100mm deep. First convert 100mm to 0.10m. The net volume is 4 × 3 × 0.10 = 1.20m³. Add the calculator's 10% allowance: 1.20 × 1.10 = 1.32m³.

If a 20kg bag yields 0.011m³, divide 1.32 by 0.011 to get 120 bags. At £6.25 each, bagged material would be 120 × £6.25 = £750. A supplier quoting £145 per m³ may impose a 1.5m³ minimum, making the concrete £217.50 before delivery, short-load or pump charges. Comparing the complete delivered price, not merely the rate, shows why ready-mix commonly wins for this size.

The calculation covers concrete only. A real slab budget may also need compacted sub-base, a damp-proof membrane, mesh reinforcement, shuttering timber and spoil removal. Price these separately so the result is not mistaken for the finished-project cost.

What drives the cost?

• Specified strength and mix design. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Slab depth, reinforcement and ground conditions. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Order size and supplier short-load charges. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Access for a mixer truck or concrete pump. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.
• Excavation, formwork and finishing labour. Ask whether this is included in a price and what assumption has been made. A change here can affect material quantity, labour time and the supporting work, so it deserves its own budget line rather than being hidden in a general contingency.

Project scale also changes the unit cost. A contractor still has travel, setup and administration on a small job, while larger orders may attract delivery efficiencies. Geography matters too: labour and parking are commonly higher in London and parts of the South East, while remote deliveries can carry their own premium. Seasonal demand may change lead times even where the quoted rate is stable.

Wastage allowance

Use 10% for a straightforward slab because excavated ground is rarely perfectly flat and shuttering can bow. Deep, irregular footings may justify 12–15%. For a controlled formed structure, 5–8% may be adequate. Concrete cannot be bought halfway through a continuous pour without risking a cold joint, so a modest surplus is normally safer than a shortage.

Apply waste to quantity before rounding to whole packs, boards, bags or rolls. Rounding first and then adding a percentage can either overbuy or leave an unusable fraction. If unopened stock is returnable, check the return window and batch restrictions. Do not assume every surplus item can be returned: special orders, mixed products and cut lengths are commonly excluded.

Measure and plan before buying

Draw the area and label every dimension instead of relying on one length and width from memory. Split L-shaped or stepped spaces into rectangles, calculate each one, and add the results. Measure in the same unit throughout. Recheck the narrowest access, floor levels, corners and fixed obstacles because these can determine product size, installation sequence or whether specialist equipment is needed.

Read the technical sheet before substituting one product for another. Two items sold for the same purpose may have different coverage, fitting centres, thicknesses or base requirements. Check compatibility between the main material and its fixings, adhesive, membrane or substrate. Manufacturer instructions normally take priority over a general calculator and may be required to preserve a warranty.

DIY budget versus a fitted quote

A DIY total should include consumables, personal protective equipment, tool hire, delivery and disposal, not only the finish material. It is sensible to keep a separate contingency of around 5–10% of the complete budget for discoveries and price movement. That contingency is different from material waste: wastage buys enough physical product, while contingency protects the cash budget.

For professional work, seek an itemised written quote. It should identify preparation, material specification, quantity, waste ownership, access assumptions, protection, cleanup, disposal, VAT, payment stages and guarantee. Ask what would count as a variation. Comparing itemised scopes is more reliable than choosing the lowest total where important work may be omitted.

Using this estimate responsibly

The result is a planning estimate, not a structural design or site survey. Hidden defects, moisture, unsuitable ground and regulated work cannot be diagnosed from dimensions alone. Confirm structural, fire, electrical, ventilation and planning requirements with a suitably competent person where they apply. Stop and investigate if the existing surface is damp, unstable or contaminated.

Save the exact product details used in the calculation, including pack coverage and unit price. When a quote or retailer price changes, update that input rather than altering the measured area. This makes the estimate auditable and helps distinguish a genuine price change from a quantity error.

Frequently asked questions