Free UK construction calculators Metric and imperial 2026 UK prices
Work out how many 10 kg, 20 kg or 25 kg bags of ready-mixed concrete you need for a slab, shed base, fence posts, footing, column or steps. Enter your sizes in metric or imperial. You get the bag count, total weight, water and cost, plus a drawing of your job.
Bags To Buy
0× 25 kg bags
Check the measurements above. The result updates once they look right.
| Bag | Bags Needed | Per m³ | Weight |
|---|
This concrete bag calculator turns your measurements into a bag count you can take to the builders' merchant. It takes about a minute.
The calculator always rounds the bag count up. Running out halfway through a pour leaves a weak joint, so a spare bag is cheap insurance.
A bag of ready-mixed concrete holds everything you need except water. Inside is a dry blend of cement, sand and gravel, already measured out in the right amounts. You tip it into a mixer or barrow, add clean water and mix. That is why it suits small jobs so well: there is no need to buy cement and ballast separately and get the ratio right yourself.
Builders' merchants and DIY stores sell several bags that look alike, so it pays to check the label before you load up the trolley.
| Bag says | What is inside | Use it for | Use this calculator? |
|---|---|---|---|
| Concrete mix / ready-mixed concrete | Cement, sand and gravel | Slabs, bases, footings, steps, post holes | Yes |
| Post mix / rapid-set post fix | Fast-setting cement and aggregate | Setting fence and gate posts quickly | Yes, but check the yield on the pack |
| Cement | Cement powder only | Mixing your own concrete with ballast | No, you also need ballast |
| All-in ballast | Sand and gravel, no cement | Mixing your own concrete with cement | No |
| Mortar mix | Cement and fine sand, no gravel | Bricklaying, pointing, bedding slabs | No, mortar is not concrete |
If you would rather mix from loose materials, a common general-purpose site mix is about 1 part cement to 5 parts all-in ballast by volume. Our ballast calculator works out how much of each you need. For a handful of bags, though, ready-mixed concrete saves time and gives a more consistent result.
Bagged ready-mixed concrete is a dry blend of cement, sand and gravel. Once you add water and compact it, roughly 2,100 kg of dry mix fills one cubic metre. That gives the figures below. They are before any waste is added.
| Bag size | Yield per bag | Bags per 1 m³ | Bags per 0.5 m³ | Bags per 0.1 m³ |
|---|---|---|---|---|
| 10 kg | 4.8 litres | 210 | 105 | 21 |
| 20 kg | 9.5 litres | 105 | 53 | 11 |
| 25 kg | 11.9 litres | 84 | 42 | 9 |
Different brands vary a little. Some 25 kg packs state a yield of about 0.011 m³ and others about 0.012 m³. Always check the back of the bag from your supplier, such as Tarmac or Blue Circle, and type that yield into the calculator if it differs.
For a flat area such as a shed base, patio pad or hot tub base, it helps to know how many bags each square metre needs at a given thickness. Multiply the figure by your area in m², then add waste.
| Thickness | Volume per m² | 25 kg bags per m² | 20 kg bags per m² | Typical use |
|---|---|---|---|---|
| 50 mm | 0.05 m³ | 4.2 | 5.3 | Path topping, small pads |
| 75 mm | 0.075 m³ | 6.3 | 7.9 | Small shed base, paths |
| 100 mm | 0.1 m³ | 8.4 | 10.5 | Shed and garden room bases |
| 150 mm | 0.15 m³ | 12.6 | 15.8 | Heavier bases, light vehicle use |
A base this size often sits on a compacted sub-base of MOT Type 1. You need that layer as well, but it is not concrete and is not counted here. Use our aggregate calculator to work out the tonnes of sub-base for the same area.
Most people searching for a concrete bag calculator have one of a handful of jobs in mind. The table below shows typical sizes and the bag count our calculator gives for each one, with a 10% waste allowance already added. Use it as a quick sense check, then enter your own measurements above for an exact figure.
| Job | Typical size | Volume with waste | 25 kg bags | 20 kg bags | Weight (25 kg bags) |
|---|---|---|---|---|---|
| Small shed base | 1.8 m × 1.2 m × 75 mm | 0.18 m³ | 15 | 19 | 375 kg |
| Standard shed base | 2.4 m × 1.8 m × 100 mm | 0.48 m³ | 40 | 50 | 1,000 kg |
| Large shed or garden room base | 3 m × 2.4 m × 100 mm | 0.79 m³ | 67 | 84 | 1,675 kg |
| Hot tub base | 2.2 m × 2.2 m × 100 mm | 0.53 m³ | 45 | 56 | 1,125 kg |
| Greenhouse pad | 2.5 m × 1.9 m × 75 mm | 0.39 m³ | 33 | 42 | 825 kg |
| Garden path | 5 m × 0.9 m × 75 mm | 0.37 m³ | 32 | 39 | 800 kg |
| One fence post | 300 mm hole, 600 mm deep, 100 mm post | 0.04 m³ | 4 | 5 | 100 kg |
| Ten fence posts | Same holes as above | 0.40 m³ | 34 | 43 | 850 kg |
| Garden wall footing | 5 m × 450 mm × 225 mm deep | 0.56 m³ | 47 | 59 | 1,175 kg |
| Three garden steps | 1 m wide, 150 mm rise, 250 mm going | 0.25 m³ | 21 | 26 | 525 kg |
Look at the weight column before you set off to collect the bags. Even a standard shed base comes to around a tonne of concrete, which is far more than most cars can carry in one trip. If the footing is for a garden wall, the block calculator tells you how many blocks to order for the wall itself. Laying a lawn around a new base or path? Our artificial grass calculator covers that part of the garden.
Every job type is worked out the same way. First find the volume in cubic metres, then add waste, then divide by how much one bag makes.
Volume = length × width × depth. Turn the depth into metres first, so 100 mm becomes 0.1 m.
Volume per hole = π × radius² × depth, minus the space the post takes up (post width² × depth). Multiply by the number of holes.
Volume = length × width × concrete depth. Use the depth of concrete, not the full dig depth.
Volume = π × radius² × height × number of columns.
Each step is a solid block. Volume = width × rise × going × n × (n + 1) ÷ 2, where n is the number of steps.
These use the same maths as the calculator above, with 10% waste and the typical yield.
Volume = 3 × 2.4 × 0.1 = 0.72 m³. Add 10% and you get 0.792 m³. One 25 kg bag makes about 11.9 litres, so 0.792 ÷ 0.0119 = 66.5. Round up to 67 bags of 25 kg, weighing 1,675 kg. With 20 kg bags you would need 84. At this size it is worth getting a ready-mix quote too.
Each hole is 300 mm across and 600 mm deep, with a 100 mm square post. One hole holds π × 0.15² × 0.6 = 0.0424 m³. The post takes up 0.1 × 0.1 × 0.6 = 0.006 m³, leaving 0.0364 m³. Six holes need 0.2185 m³, or 0.2403 m³ with 10% waste. That is 21 bags of 25 kg or 26 bags of 20 kg.
The steps are 1 m wide, with a 150 mm rise and a 250 mm going. Volume = 1 × 0.15 × 0.25 × 3 × 4 ÷ 2 = 0.225 m³. With 10% waste it is 0.2475 m³, which needs 21 bags of 25 kg or 26 bags of 20 kg.
A bag count is only as good as the measurements behind it. A few millimetres of extra depth across a whole base can add several bags, so it is worth taking a little care before you type the numbers in.
Ground is rarely perfectly level. Check the depth from the top of your formwork to the sub-base at each corner and in the middle, then use the average. If one end is much deeper than the other, measure along the length every metre or so and average all of the readings.
For an L-shaped base or a path with a turn, divide the area into simple rectangles. Work out each one with the slab option, then add the bag counts together. For a round pad, use the column option with a height equal to the slab depth and a count of one.
Take your length and width from the inside faces of the timber shuttering, not the outside. The thickness of the boards does not hold any concrete, and measuring to the outside can overstate a small base by a surprising amount.
Holes dug by hand or with an auger often end up wider at the top than the bottom. Measure the diameter at the top and halfway down and use a figure between the two. If the sides crumbled while you were digging, choose the 15% waste option.
Both sizes hold the same mix. The choice comes down to handling, price and what your local merchant stocks.
For guidance on lifting heavy sacks safely, see the HSE manual handling advice. Rapid-set post mixes are sold in similar sizes but can yield a little differently. Use the yield box if your pack shows one.
Bags are ideal for small jobs. As the volume grows, mixing by hand gets slow and the cost per cubic metre climbs. Use these rough limits:
When you ask for a ready-mix quote, give the supplier the "volume to buy for" from the calculator result. That figure already includes waste.
Builders' merchants and the large DIY chains in the UK stock ready-mixed concrete in 20 kg and 25 kg bags. Trade counters tend to favour 25 kg bags, while DIY stores often sell smaller packs and multi-buy deals. For bigger orders, ask the merchant about a full pallet delivered to your drive. The number of bags on a pallet varies by supplier, so check before you order.
Every car has a maximum payload, listed in the handbook, that covers passengers as well as cargo. Twenty 25 kg bags weigh 500 kg, which is at or above the limit for many family cars. The calculator shows how many trips you would need at around 400 kg a load. If the total runs into tonnes, delivery is safer and usually saves money on fuel and time.
Real ground is never perfectly flat, and some concrete is always lost in the mixer, the barrow and on the boots. A waste allowance covers this.
| Waste | When to use it |
|---|---|
| 5% | Timber formwork on a well-levelled, compacted sub-base |
| 10% | Most slabs, footings and steps. The default in this calculator |
| 15% | Hand-dug post holes, rough trenches, soft or uneven ground |
Concrete is sold by the cubic metre in the UK, but plenty of plans and older tape measures still use feet and inches. The calculator converts for you when you press Imperial. If you are working by hand, these are the figures you need.
| Unit | Equals | Handy to know |
|---|---|---|
| 1 metre | 3.281 feet | A 3 m base is just under 10 ft |
| 1 foot | 0.3048 metres | Multiply feet by 0.3048 to get metres |
| 1 inch | 25.4 millimetres | A 4 in slab is about 100 mm |
| 100 millimetres | 3.94 inches | The most common shed base depth |
| 1 cubic metre | 35.31 cubic feet | About 84 bags of 25 kg |
| 1 cubic metre | 1.308 cubic yards | US and older plans often use yards |
| 1 cubic yard | 0.7646 cubic metres | About 64 bags of 25 kg |
| 1 cubic metre | 1,000 litres | A 25 kg bag makes about 11.9 litres |
| 1 square metre | 10.76 square feet | Useful when pricing by area |
Good mixing matters as much as the right number of bags. A well-mixed batch is easier to place, finishes better and ends up stronger.
Aim for a consistency like thick porridge. If you squeeze a handful in a gloved hand it should hold together without water running out. Too wet and the concrete will shrink and crack; too dry and it will not compact properly around posts or into the corners of your formwork.
Check whether your project needs planning permission or building regulations approval before you dig. The Planning Portal covers outbuildings, garden walls and more.
Concrete does not dry like paint. It hardens through a chemical reaction between the cement and the water, and that reaction carries on for weeks. Keeping it damp for the first few days helps it reach full strength. The times below are typical for standard bagged concrete in mild weather. Cold weather slows everything down, so always follow the instructions on your pack.
| Stage | Typical time | What you can do |
|---|---|---|
| Initial set | A few hours | Surface starts to stiffen; finish the top before this |
| Light foot traffic | About 24 to 48 hours | Walk on it carefully and remove formwork |
| Normal use | About 7 days | Build a shed on the base or set up a hot tub frame |
| Close to full strength | About 28 days | Heavier loads such as a car on a thicker slab |
| Rapid-set post mix | Minutes rather than hours | Hold the post upright; check the pack for exact times |
About 84 bags of 25 kg make 1 m³, using a typical yield of 11.9 litres per bag. Add your waste allowance on top. With 10% waste you would buy about 93 bags.
About 105 bags of 20 kg make 1 m³. Each 20 kg bag gives about 9.5 litres of concrete.
One 25 kg bag makes about 0.0119 m³. At 100 mm thick that covers about 0.12 m². At 50 mm thick it covers about 0.24 m².
A 300 mm wide, 600 mm deep hole with a 100 mm post needs about 0.036 m³. That is about 3 to 4 bags of 25 kg per post, depending on your waste allowance. Use the fence post option above for your exact sizes.
A 3 m × 2.4 m base at 100 mm thick needs 67 bags of 25 kg with 10% waste. Smaller sheds need less. Enter your shed size above for an exact count.
Bags are usually best below about 0.5 m³. Above about 1 m³, ready-mix or a volumetric truck is normally cheaper per m³ and much faster. In between, compare quotes.
Follow the instructions on your bag. As a rough guide, a 25 kg bag needs around 2 to 3 litres. Add it bit by bit until the mix is workable but not runny.
Yes. Switch to Imperial and enter long sizes in feet and thickness in inches. The result still shows cubic metres, plus cubic feet and cubic yards.
You cannot buy part of a bag, and running short mid-pour is worse than having one spare. The result shows the exact figure before rounding so you can see how close it is.
No. The count is for concrete only. Hardcore or MOT Type 1 under a slab is a separate material.
Standard bagged concrete usually starts to stiffen within a few hours and can take careful foot traffic after about a day or two. Leave a base for around a week before heavy use. It keeps gaining strength for about four weeks. Rapid-set post mixes set much faster, so follow the pack.
Both contain cement and aggregate. Post mix is made to set very quickly so a fence post stays upright without propping. Standard concrete mix gives you more working time, which suits slabs, footings and steps.
No. Mortar has fine sand but no gravel, so it is not strong enough for slabs, bases or post holes. It is made for laying bricks, pointing and bedding paving.
Avoid pouring when frost is forecast or temperatures are near freezing, as ice can damage the concrete before it sets. In rain, cover fresh concrete with a sheet so the surface is not washed out.
It depends on your car's payload, which is in the handbook and includes passengers. Twenty 25 kg bags weigh 500 kg, which is at or over the limit for many cars. The calculator assumes about 400 kg per trip.
For up to about 20 bags you can mix by hand in a wheelbarrow. Above that, hiring a mixer saves a lot of effort and gives a more even mix.
You can, but the gravel can settle to the bottom of the bag. Turn the dry contents over first so each half has the same mix. Mixing whole bags gives the most consistent result.
Planning more of the job? These tools work alongside the concrete bag calculator.
Figures use a typical placed density of 2,100 kg of dry bagged mix per m³. Product yields vary, so always check the pack. Reviewed November 2026. See our Disclaimer.