Concrete calculator

DIY complexity

Advanced

Mixing, placing and finishing concrete before it sets is physically demanding and time-critical — small slabs and footings are DIY-feasible, but getting the timing, formwork and finish right takes experience.

Introduction

This calculator estimates the volume of concrete — and the number of ready-mixed bags or the ready-mix (truck) order — needed for a slab, a strip or pad footing, or a post hole. Because volume is simply plan area multiplied by depth, a small error in depth has an outsized effect on the final quantity: doubling the depth doubles the volume, and even a centimetre of error on a thin slab noticeably shifts the bag count. The supported modes differ mainly in how their plan area is defined — a slab uses length and width, a strip footing uses trench length and width, and a post hole uses a diameter — but all three share the same depth-and-waste logic once that area is known. Reinforcement such as rebar or welded mesh is not sized or counted by this tool; treat it as a separate structural line item and consult an engineer or the local building code for bar diameter, spacing and cover. For purchasing, the result can be expressed either as a number of bags at a given yield, or compared against a ready-mix truck delivery, which is worth checking against your supplier's minimum order quantity before choosing between bags and a truck.

Quick answer

For a 6 × 3 m terrace slab at 12 cm depth with 10% waste, the concrete volume is about 2.38 m³ (6 × 3 × 0.12 × 1.10); at an example yield of 0.0125 m³ per 25 kg bag that is roughly 191 bags, but always verify the yield on the selected bag's data sheet.

Calculation methodology

01Choose the mode — slab, strip/pad footing, or post hole — and enter its plan dimensions (length and width, or diameter for a hole).
02Enter the depth in the selected unit, converting mm/cm/m consistently — a unit mistake here distorts the whole result.
03Multiply plan area by depth to get the theoretical net volume before waste.
04Add the chosen waste percentage to cover subgrade unevenness, over-excavation, spillage and formwork loss.
05Convert the volume including waste into whole bags using the selected bag yield, rounding up, or compare it against a ready-mix truck order.
06Add reinforcement (rebar/mesh) as a separate, manually priced line item — it is not sized or counted by this calculator.

Formulas

volume = plan_area × depth × (1 + waste)

The theoretical volume comes from multiplying the plan area (from length and width, or a hole's diameter) by the pour depth, then inflating it by the waste percentage to allow for subgrade unevenness and spillage.

Variables

plan_area = footprint area of the slab, footing or hole (m²)
depth = pour thickness or hole depth (m)
waste = allowance for spillage, over-excavation and formwork loss (%)
volume = concrete volume to place (m³)

Rounding rule: The volume itself is shown rounded to two decimals for readability; rounding up to whole purchase units only happens at the bag-count step.

Limitation: Assumes a uniform depth across the whole poured area; a stepped footing or a slab with a thickened edge must be split into separate zones and summed.

bags = ceil(volume ÷ bag_yield)

The volume including waste is divided by how much usable concrete one bag of the selected size and mix yields once mixed with water, then rounded up to a whole bag.

Variables

volume = concrete volume including waste (m³)
bag_yield = usable mixed volume produced by one bag (m³ per bag)
bags = whole bags to purchase

Rounding rule: Always rounded up (ceiling) — a pour cannot use a fractional bag.

Limitation: Bag yield varies by bag weight, mix design and manufacturer; 0.0125 m³ per 25 kg bag used in examples on this page is an editable illustration, not a universal constant — use the figure on the selected bag's data sheet. For large volumes, compare the bag total against a ready-mix truck's price and minimum order rather than assuming bags are always cheaper.

Inputs that matter

DepthVolume scales directly with depth, so a small measurement or unit-entry error here changes the final quantity more than almost any other input.
Mode (slab, footing, post hole)Determines how plan area is calculated — length × width for a slab or footing, or a circular area from diameter for a hole.
Length and width (or diameter)Sets the base plan area that depth is multiplied against; footing width is easy to underestimate since trenches are rarely perfectly straight-sided.
Waste allowanceCovers subgrade unevenness, over-excavation, spillage and formwork loss — footings dug by hand typically need more waste than a screeded slab on flat ground.
Bag yieldThe usable mixed volume from one bag varies by bag weight and mix design; using the wrong yield skews the bag count more than any other purchasing input.
Bags vs ready-mix (truck)Large volumes are usually cheaper and faster by ready-mix truck, but suppliers apply a minimum order quantity that a small pour may not reach.

Worked examples

Terrace slab, 6 × 3 m at 12 cm depth

A garden terrace slab: 6 m long by 3 m wide, 12 cm depth, 10% waste, purchasing bags with an example yield of 0.0125 m³ per 25 kg bag.

plan_area = 6 × 3 = 18 m². Net volume = 18 × 0.12 = 2.16 m³. With 10% waste: 2.16 × 1.10 = 2.376 → 2.38 m³. bags = ceil(2.376 ÷ 0.0125) = ceil(190.1) = 191 bags.

2.16 m³ is the theoretical net volume; 2.38 m³ is the quantity including waste; 191 bags is the final purchase quantity. Reinforcement mesh, if used under a terrace slab, is a separate line item this tool does not size.

⚠ The 0.0125 m³ per bag yield is an editable example — check the actual yield printed on the selected bag's packaging or data sheet before ordering.

Vehicle-bearing garage slab, 5 × 5 m at 15 cm depth

A reinforced garage slab designed to carry vehicle loads: 5 m × 5 m, 15 cm depth, 10% waste, same example bag yield. Reinforcing mesh is planned but priced and sized separately.

plan_area = 5 × 5 = 25 m². Net volume = 25 × 0.15 = 3.75 m³. With 10% waste: 3.75 × 1.10 = 4.125 → 4.13 m³. bags = ceil(4.125 ÷ 0.0125) = 330 bags exactly.

At 4.13 m³, ready-mix truck delivery is often more practical than 330 hand-mixed bags — check your supplier's minimum order. Because this slab carries vehicle loads, reinforcing mesh sizing and cover depth should come from an engineer or the applicable local code, not from this calculator.

⚠ This calculator produces quantities only — it does not check whether 15 cm depth and the planned mesh are adequate for the expected vehicle load; that is a structural design question for an engineer.

Strip footing, 12 m long: bags versus ready-mix truck

A 12 m long, 0.5 m wide strip footing at 24 cm depth, 10% waste. The example compares purchasing bags against ordering a ready-mix truck, which typically has a minimum order quantity.

plan_area = 12 × 0.5 = 6 m². Net volume = 6 × 0.24 = 1.44 m³. With 10% waste: 1.44 × 1.10 = 1.584 → 1.58 m³. bags = ceil(1.584 ÷ 0.0125) = 127 bags (about 3.18 tonnes of bagged mix). At an example supplier minimum order of 1 m³, a 1.58 m³ ready-mix order clears the minimum easily.

127 bags mixed by hand is a lot of manual labour for a single footing; comparing the bagged-mix cost against a ready-mix truck's price per m³ plus delivery fee often changes the decision, especially once the minimum order is already exceeded.

⚠ Changing the selected currency only changes the display symbol — it does not convert prices you have entered for bags or for a ready-mix truck. Enter current local supplier prices in the currency you have selected, and confirm the actual minimum order quantity with the supplier, since it is not a fixed universal figure.

Reading the results

The result panel separates the theoretical net volume, the volume including waste, and the final whole-bag purchase quantity (or the ready-mix comparison) — each figure is larger than the last for a specific reason, not a rounding inconsistency. Where reinforcement is planned, it appears as a reminder line rather than a calculated quantity, because bar diameter, spacing and cover depend on structural design, not on the geometry entered here. When a bag price is entered, the total cost reflects the whole-bag purchase quantity, not the theoretical volume, since concrete cannot be bought in fractional bags.

Common mistakes

Entering depth in the wrong unit (mixing mm, cm and m), which distorts the volume by a factor of 10 or 100.
Assuming the subgrade or trench base is perfectly level, when real ground variation is exactly what the waste allowance exists to cover.
Forgetting that reinforcement (rebar or mesh) is a separate structural line item this calculator does not size or count.
Confusing a bag's total weight with its usable mixed yield — the two are not the same figure.
Ordering exactly the calculated volume from a ready-mix truck without checking the supplier's minimum order quantity or rounding increment.
Under-allowing waste for a hand-dug footing or post hole, where irregular trench walls and soil collapse consume noticeably more concrete than a formed, screeded slab.

Adapting to your country

Bagged concrete mix is typically sold in 20, 25 or 40 kg bags depending on the country and manufacturer, with the usable mixed yield printed on the bag rather than following a fixed international figure — always read it off the selected product before finalising a bag count. Ready-mix truck concrete is priced per m³ (or per cubic yard in imperial markets) and almost always carries a minimum order quantity and a delivery or short-load surcharge below that minimum, both of which vary by supplier and region. Metric markets measure depth in mm or cm and volume in m³; imperial markets commonly use inches for depth and cubic yards for volume. Selecting a different currency only changes the displayed symbol — enter a current local supplier price for an accurate cost estimate, and check applicable sales tax separately, since it is not built into the calculator's cost output.

Limitations and verification

This calculator produces a planning estimate of concrete volume and purchase quantity, not a structural design, building-permit submission or contractor quotation. It does not size reinforcement (rebar diameter, spacing, cover), verify load-bearing capacity, check frost depth or drainage requirements, or confirm compliance with local building regulations. Complex or stepped geometry should be split into separate zones and summed, and any project involving vehicle loads, structural spans or foundations for a building should be reviewed by a qualified engineer before construction. Always confirm final quantities against actual site measurements and the selected product's current data sheet before ordering.

Last reviewed: 2026-08-24

Frequently asked questions

Why does a small depth error change the result so much?

Volume is plan area multiplied directly by depth, so any error in depth carries through at full strength — a 1 cm error on a 10 cm slab is a 10% volume error, which a flat error in length or width of the same size would not produce.

What is the difference between a slab, a footing and a post hole in this calculator?

They differ only in how plan area is defined — length × width for a slab or strip/pad footing, and a circular area from diameter for a post hole — after which the same depth-and-waste logic applies to all three.

Does this calculator size the rebar or mesh I need?

No. Reinforcement diameter, spacing and cover depend on structural loading and local code requirements; this tool only calculates concrete volume and purchase quantity, and reinforcement should be priced and sized as a separate item.

What is bag yield and why does it vary?

It is the usable mixed volume produced by one bag once combined with water, and it varies by bag weight and mix design — always use the figure from the selected bag's data sheet rather than a generic assumption.

When should I order ready-mix (a truck) instead of bags?

Ready-mix is usually more practical once the required volume is large enough to clear the supplier's minimum order, since mixing hundreds of bags by hand is slow and labour-intensive; check the minimum order with your local supplier.

How much waste allowance should I use?

There is no single correct percentage — a formed slab on flat, prepared ground typically needs less waste than a hand-dug footing or post hole with irregular trench walls; adjust the default upward for rougher ground conditions.

Does changing currency convert the bag or truck price?

No. Changing currency only changes the displayed symbol and formatting; enter a current local supplier price in the currency you select.

Is this result enough to order materials or get a permit?

No. It is a planning estimate for quantities only. Verify reinforcement requirements, minimum depths and any local building regulations with an engineer or the relevant authority before ordering or applying for approval.

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