How do I calculate concrete for a slab?

Concrete slab volume = length x width x thickness. Convert thickness to the same unit as length and width before multiplying. For a 4m x 5m slab at 10cm thickness: 4 x 5 x 0.10 = 2.0 m3. Add 10% waste: 2.0 x 1.10 = 2.2 m3. At 0.011 m3 per 25kg bag, you need approximately 200 bags of 25kg concrete.

What concrete mix ratio should I use?

Standard mix (1:2:3 cement:sand:aggregate) is suitable for general purpose use, paths and non-structural slabs. Strong mix (1:1.5:3) for driveways, structural slabs and footings exposed to heavy loads. Lean mix (1:3:6) for mass fill, blinding and non-structural applications. Floor mix (1:2:4) for floor slabs and internal paths. Pre-bagged concrete (Readymix) includes all components in the correct proportions.

Concrete Calculator – Volume & Bags for Any Shape

Q: How much concrete do I need?

Select your shape type, enter the dimensions and click Calculate. The tool computes the volume in m3 (metric) or ft3/yd3 (imperial), adds a waste factor (10% by default) and shows the number of bags needed for your chosen bag size. For a standard 3m x 3m slab at 10cm thickness, you need approximately 0.99 m3 of concrete including 10% waste.

Q: How many bags of concrete do I need?

Divide total volume (with waste factor) by the yield per bag. A 25kg bag yields approximately 0.011 m3. A 50kg bag yields approximately 0.022 m3. A 60lb bag yields approximately 0.45 ft3. A 80lb bag yields approximately 0.60 ft3. The calculator does this automatically once you enter your dimensions and choose a bag size.

Q: What is the 10% waste factor?

Concrete waste occurs due to spillage during pouring, overfilling forms, irregular sub-base absorption and unavoidable mixing losses. A 10% waste factor is the standard addition recommended for most DIY projects. Add 15-20% for complex shapes, uneven ground, or projects where ordering additional concrete would cause delays. The waste slider in the calculator lets you adjust from 0 to 30%.

Q: How many cubic yards of concrete do I need?

Switch to imperial units. The calculator shows volume in both cubic feet (ft3) and cubic yards (yd3). Ready-mix concrete is typically sold by the cubic yard. 1 cubic yard = 27 cubic feet. For a 10ft x 12ft slab at 4 inches thick: 10 x 12 x (4/12) = 40 ft3 = 1.48 yd3. With 10% waste: 1.63 yd3. Most suppliers require a minimum order of 1 yd3.

Q: Concrete calculator for post holes

Use the Round Slab shape or Column shape. Enter the diameter and depth of the post hole. The calculator computes the cylindrical volume using pi x radius squared x depth. Add 15-20% waste for post holes as the exact volume is difficult to control. A standard 300mm diameter hole at 600mm depth requires approximately 0.042 m3 of concrete.

Q: How do I calculate concrete for steps?

Select the Steps shape. Enter the stair width, rise (height of each step), run (depth of each step) and number of steps. The calculator uses the staircase volume formula: width x rise x run x n(n+1)/2, which accounts for the triangular cross-section of a standard concrete staircase. Add 15% waste for steps as forming and finishing involve more waste than flat pours.

How It Works

Six Shape Types — One Concrete Calculator

Most concrete calculators only handle rectangular slabs. Real projects involve footings, columns, round pads, retaining walls and steps — each with a different volume formula. This calculator covers all six common shapes with the correct geometry for each.

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Rectangular slab — length × width × thickness — The most common pour. Used for driveways, patios, shed bases, garage floors and pathways. Standard residential pedestrian slabs are 10cm (4 inches) thick. Driveways and vehicular pads need 15cm (6 inches) minimum. A 4m × 4m slab at 10cm thickness requires approximately 1.76 m³ with 10% waste included.

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Round / circular slab — π × radius² × thickness — Used for circular pads, garden features, hot tub bases and round pool surrounds. Enter the full diameter, not the radius — the calculator halves it automatically. A 2.4m diameter round pad at 10cm thickness uses approximately 0.498 m³ before waste.

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Wall — length × height × thickness — Retaining walls, garden walls, boundary walls and concrete block fill. Enter the wall face dimensions (length and height) and the wall thickness. Standard concrete block walls are 10–20cm thick. This formula calculates solid concrete volume — if pouring into hollow blocks, only a fraction of that volume fills the cores.

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Column / post hole — π × radius² × depth — Cylindrical columns, fence post holes, telegraph poles and structural posts. Enter the hole diameter and depth. A standard 100mm (4-inch) diameter fence post in a 300mm diameter hole at 600mm depth uses approximately 0.042 m³ per post. Add 15% waste for post holes as exact filling is difficult.

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Footing / pad foundation — length × width × depth — Strip footings, pad footings and mass foundations. UK Building Regulations typically specify strip footings at 450mm wide × 225mm deep for standard domestic construction. Deeper footings may be required for two-storey buildings, poor ground conditions or proximity to trees. Add 15% waste for footings.

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Steps / staircase — width × rise × run × n(n+1)/2 — The staircase formula accounts for the triangular cross-section that accumulates at each step. A 4-step staircase poured as a monolithic concrete structure has significantly more volume than 4 individual rectangular steps because the back fill accumulates. Standard step dimensions: 16–18cm rise, 25–30cm run, minimum 90cm width.

Project Guide

Bagged Concrete vs Ready-Mix — When to Use Which

When bagged concrete makes sense

Bagged concrete (pre-mixed dry concrete in paper or plastic sacks) is ideal for small jobs up to approximately 0.5–1 m³ (18–35 ft³ or about 0.65–1.3 yd³). The advantages are simplicity — no minimum order, no truck access required, pour at your own pace — and the ability to mix only what you need, avoiding waste on tight jobs. Common bagged applications include fence post holes, small repair pours, step repairs, garden walls and small pad footings. The trade-off is cost per m³: bagged concrete typically costs 3–5x more per cubic metre than ready-mix when bought in small quantities.

When ready-mix is the better choice

Ready-mix concrete delivered by a transit mixer (drum truck) becomes cost-effective at volumes above approximately 1 m³ (1.3 yd³). The concrete is precisely batched to your specified strength class (C20, C25, C30 for UK; 3000 psi, 4000 psi for US) and arrives ready to pour. Most suppliers require a minimum order of 1 m³ (or 1 yd³ in the US). Ready-mix is essential for driveways, garage floors, structural foundations and any pour where consistency of mix is critical. The concrete calculator shows your volume in cubic yards — the unit suppliers use — when imperial units are selected.

Understanding the waste factor

The 10% default waste factor accounts for spillage during pouring, uneven sub-base absorption (concrete flows into low spots), overfilling forms and the concrete remaining in the mixer or barrow after pouring. Increase to 15% for complex shapes with curved forms, uneven ground or interrupted pours. Use 20% for staircase pours, curved retaining walls and any situation where the exact volume is difficult to control. Do not set waste to 0% — even the most controlled pour has some unavoidable loss. Running short of concrete mid-pour creates cold joints — weak seams in the finished surface where fresh concrete bonds imperfectly to concrete that has already begun setting.

Mix Guide

Concrete Mix Ratios — Choosing the Right Strength

Concrete mix ratio refers to the proportions of cement, sand (fine aggregate) and coarse aggregate by volume or weight. Different applications require different strengths, achieved by varying the cement content relative to the aggregates.

Mix	Ratio (C:S:A)	Strength class	Best applications
Standard	1 : 2 : 3	C20 / 3000 psi	Garden paths, non-structural slabs, fence posts, general fill
Strong / structural	1 : 1.5 : 3	C25 / 3500 psi	Driveways, structural slabs, footings, columns, lintels
Floor mix	1 : 2 : 4	C20 / 3000 psi	Internal floor slabs, pathways, shed bases
Lean / blinding	1 : 3 : 6	C10 / 2000 psi	Mass fill, blinding layer under footings, non-structural

What the ratios mean in practice

A 1:2:3 mix means 1 part cement to 2 parts sand to 3 parts aggregate by volume. For a 25 kg bag of cement, you would use 50 kg of sand and 75 kg of gravel. In practice, most DIY users buy pre-mixed dry bagged concrete rather than batching their own, as the proportions are already balanced. Pre-bagged concrete is equivalent to a standard 1:2:3 mix for general-purpose bags, or 1:1.5:3 for high-strength variants (labelled “Postcrete”, “Rapid Set” or “Quikrete 5000” depending on brand and region).

Comparison

LazyTools vs Other Concrete Calculators

Feature	LazyTools	Calculator.net	Omni Concrete	Quikrete.com
Rectangular slab	✅ Yes	✅ Yes	✅ Yes	✅ Yes
Round slab	✅ Yes	✅ Yes	✅ Yes	❌ No
Steps / staircase	✅ Yes	⚠ Basic	✅ Yes	❌ No
Bag size selector	✅ 4 sizes + brands	⚠ US only	⚠ Limited	✅ Yes (Quikrete)
Metric + imperial	✅ Both	✅ Both	✅ Both	⚠ Imperial only
Adjustable waste factor	✅ 5–20%	❌ Fixed	✅ Yes	❌ None
Mix ratio guide	✅ 4 mixes	❌ No	⚠ Article only	❌ No

Reference

Standard Concrete Slab Thickness by Application

Application	Metric thickness	Imperial thickness	Notes
Garden path (pedestrian)	7.5–10 cm	3–4 in	Light foot traffic only
Patio / terrace	10 cm	4 in	Furniture, light domestic use
Shed / workshop base	10 cm	4 in	Reinforce edges for heavy machinery
Driveway (cars)	15 cm	6 in	C25 strength minimum
Driveway (vans / HGV)	20 cm	8 in	Reinforce with A142 mesh or rebar
Garage floor	10–15 cm	4–6 in	Depends on vehicle weight
Strip footing (2-storey)	45 cm wide, 23 cm deep	18 in wide, 9 in deep	Check local Building Regs

Practical Guide

How to Order and Pour Concrete — A Practical Guide for DIYers

How to calculate concrete for a driveway

A standard double driveway in the UK is approximately 5.5m wide and 5m long (about 18ft × 16ft in the US). At 15cm (6 inches) thickness required for vehicular use: 5.5 × 5 × 0.15 = 4.125 m³ before waste. With 10% waste: 4.54 m³. At this volume, ready-mix concrete is almost always more practical and economical than bagged. Most UK ready-mix suppliers deliver from 1 m³. For a driveway this size, expect 5 m³ including waste. Use a C25 or equivalent mix for domestic driveways and a higher spec for commercial vehicle access.

Concrete for fence posts — how many bags per post

The standard approach for a timber fence post is a concrete collar: a hole approximately 3× the post diameter in width and 1/3 of the post length in depth. For a 75mm × 75mm post (3 inch) in a 300mm wide hole at 600mm depth: volume = π × (0.15)² × 0.6 = 0.042 m³. Subtract the post volume (negligible at this scale). At 0.011 m³ per 25kg bag: approximately 4 bags per post. Use the Column shape in the calculator and add 15% waste for post holes as backfilling is imprecise.

Cold weather concreting — what the calculator cannot account for

The volume calculated here is purely geometric — it does not account for environmental factors that affect whether you should pour at all. Do not pour concrete when air temperature is below 5°C (41°F) or is expected to fall below freezing within 24 hours. Frost damage to fresh concrete is catastrophic — ice crystals forming in the setting concrete disrupt the cement hydration process, permanently weakening the structure. In cold weather, order a faster-setting concrete mix, insulate the freshly poured surface with polythene and insulating blankets, and extend the curing time before loading. In hot and sunny conditions (above 30°C / 86°F), wet cure the surface by covering with damp hessian to prevent premature moisture loss that causes cracking.

FAQ

Frequently Asked Questions

How much concrete do I need for a slab?▾

Volume = length x width x thickness (all in the same unit). A 3m x 4m slab at 10cm thick = 3 x 4 x 0.10 = 1.2 m3. Add 10% waste = 1.32 m3. At 0.011 m3 per 25kg bag, that is approximately 120 bags. Enter your measurements above for an instant calculation.

How many bags of concrete do I need?▾

Divide total volume (with waste) by yield per bag. 25kg bag: 0.011 m3. 40kg bag: 0.018 m3. 60lb bag: 0.45 ft3. 80lb bag: 0.60 ft3. Always round up to the nearest whole bag. The calculator does this automatically.

How many cubic yards of concrete do I need?▾

Switch to imperial units. The calculator shows volume in ft3 and yd3 (1 yd3 = 27 ft3). Ready-mix is sold by the cubic yard. A 10ft x 12ft x 4in slab = 1.48 yd3 before waste, 1.63 yd3 with 10% waste added.

What is the 10% waste factor?▾

Waste accounts for spillage, uneven ground absorption, over-filling forms and mixer residue. 10% is standard for most pours. Use 15-20% for complex shapes or uneven ground. Never set to 0% - ordering exactly to volume risks running short mid-pour, creating cold joints.

What mix ratio should I use?▾

Standard 1:2:3 for paths, garden walls, small slabs. Strong 1:1.5:3 for driveways, footings, structural slabs. Floor mix 1:2:4 for internal floors. Lean 1:3:6 for mass fill and blinding. Pre-bagged concrete already contains the correct proportions.

Concrete calculator for post holes▾

Use Column / post hole shape. Enter hole diameter (typically 3x the post width) and depth (typically 1/3 of post length). A 300mm diameter hole at 600mm depth = 0.042 m3 = about 4 bags of 25kg. Add 15% waste for post holes.

Bagged concrete vs ready-mix - which should I use?▾

Bagged: volumes under 0.5-1 m3, no truck access, no minimum order. Ready-mix: volumes over 1 m3, more cost-effective at scale, consistent quality. Ready-mix is sold by the cubic yard (or m3) - the calculator shows both units.

How do I calculate concrete for steps?▾

Select Steps shape. Enter stair width, rise (height per step), run (depth per step) and number of steps. The formula accounts for the triangular accumulation of volume at each step: width x rise x run x n(n+1)/2. Add 15-20% waste for staircase pours.

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