Concrete Bags Calculator
Calculate how many bags of premixed concrete you need from a known concrete volume, including waste allowance, supplied volume, leftover material, total weight and cost.
Assumptions
- Preset bag yields are representative estimates.
- Actual yield varies by manufacturer and product; use the manufacturer-stated yield where available.
- Bags are always rounded up to the next whole bag.
- The waste allowance is added before calculating the required number of bags.
Use this concrete bags calculator to estimate how many bags of premixed concrete you need for a known volume. Enter the required concrete volume, choose your unit, select a bag size and confirm the stated yield per bag. You can also add a waste allowance and a price per bag.
The calculator rounds the result up to a whole bag and shows the adjusted volume required, total volume supplied, expected leftover material, combined bag weight and estimated material cost. It supports litres, cubic metres, cubic feet and cubic yards, making it useful for small slabs, post holes, steps, pads, repairs and other jobs where bagged concrete is practical.
PRO TIP: Use the yield printed on the exact bag you plan to buy rather than relying on the preset. Products with the same bag weight can produce different mixed volumes. For several separate pours, calculate each area carefully and consider whether a single overall waste allowance is more realistic than adding the full allowance to every small section.
How to use this concrete bags calculator
Enter the required concrete volume
Start with the total finished concrete volume before waste. The input must be greater than zero. This calculator does not calculate volume from length, width and depth, so measure or calculate the volume separately before using it.
Choose the volume unit
Select the same unit used for your volume figure. The calculator accepts litres, cubic metres, cubic feet and cubic yards. It converts the entered amount to litres internally before calculating the number of bags.
Select the bag size and yield
Preset options are 20 kg, 25 kg and 30 kg. Their default yields are 10 litres, 12 litres and 14 litres respectively. The yield field remains editable because actual coverage varies by product. For a custom bag, enter both its weight in kilograms and its yield in litres.
Add a waste allowance
Choose none, 10% or a custom allowance between 0% and 100%. Waste is added to the original volume before the bag quantity is calculated. A modest allowance can help cover spillage, uneven excavations, small measurement errors and material left in the mixer or tools.
Add the price per bag
Enter a cost per bag in pounds to calculate an estimated material total. This field is optional and can be left blank. The calculator accepts zero or any positive amount.
Review and reset
The results include a summary and a full calculation breakdown. Changing an input clears the displayed result until you calculate again. Use Reset to return to litres, a 20 kg bag with a 10-litre yield, and a 10% waste allowance.
Results Explanation
- Bags Required: the number of whole bags to buy after applying the selected waste allowance. Any part-bag result is rounded up.
- Volume Required: the original concrete volume plus the waste allowance, shown in your selected unit and also in litres when another unit is used.
- Volume Supplied: the total nominal yield from all required bags.
- Leftover Volume: the difference between the supplied volume and the adjusted required volume.
- Total Weight: the number of bags multiplied by the selected or custom bag weight.
- Estimated Cost: the required whole bags multiplied by the entered cost per bag. A dash appears when no price is entered.
- Calculation Breakdown: the original volume, waste added, adjusted volume, bag yield, exact unrounded bag count, rounded bag count, supplied volume and leftover volume.
How the Maths Works
The calculator first converts the entered volume into litres using these conversion factors:
- 1 m³ = 1,000 litres
- 1 ft³ = 28.316846592 litres
- 1 yd³ = 764.554857984 litres
It then applies the selected waste percentage:
Waste volume = original volume × (waste percentage ÷ 100)
Adjusted volume = original volume + waste volume
The adjusted volume is divided by the stated yield of one bag:
Exact bags = adjusted volume ÷ yield per bag
The exact result is rounded up to the next whole bag. A very small floating-point tolerance is used so an exact whole-number result is not accidentally rounded up.
The remaining outputs are calculated as follows:
- Supplied volume = whole bags × yield per bag
- Leftover volume = supplied volume − adjusted volume, never shown below zero
- Total weight = whole bags × bag weight
- Estimated cost = whole bags × cost per bag
Common Mistakes to Avoid
- Entering area instead of volume. Square metres or square feet must first be converted into a three-dimensional volume using the concrete depth.
- Using the dry bag weight as the yield. Kilograms describe product weight; the calculator needs the mixed concrete yield in litres.
- Leaving the preset yield unchanged when the manufacturer states a different figure.
- Mixing units, such as entering a cubic-metre value while litres are selected.
- Forgetting irregular or oversized excavations, especially around post holes and rough ground.
- Using an excessive waste percentage without checking whether the original measured volume already includes an allowance.
- Assuming the estimated cost includes delivery, mixing equipment, water, reinforcement, formwork or labour.
- Planning a pour that is too large to mix and place consistently with bagged concrete.
faqs
How many bags of concrete do I need?
Divide the required concrete volume, including waste, by the mixed yield of one bag, then round up. This calculator performs those steps and reports the whole bags to buy.
How much concrete does a 20 kg bag make?
The calculator uses a default yield of 10 litres for a 20 kg bag, but this is only a preset. Check the product packaging or technical data because actual yields vary by manufacturer and mix.
Is bagged concrete suitable for a large slab?
Bagged concrete is often convenient for smaller jobs and repairs. Larger pours may require many bags, repeated mixing and careful timing, so ready-mixed concrete may be more practical. Consider access, placement time, consistency and labour.
How much waste should I allow for concrete?
The calculator offers no waste, 10% or a custom percentage. The appropriate allowance depends on measurement accuracy, excavation shape, spillage and site conditions. Avoid adding waste twice if your entered volume already includes it.
Why are concrete bags always rounded up?
Concrete is normally bought in whole bags, and buying a fraction of a bag is not practical. Rounding up also ensures the nominal supplied yield is at least the adjusted required volume.
What does leftover volume mean?
Leftover volume is the nominal yield supplied by the rounded whole bags minus the required volume after waste. It is an estimate because real-world yield and site losses can vary.
Does the calculator include the weight of added water?
No. Total weight is calculated from the dry bag weight multiplied by the number of bags. The supplied code does not add mixing water or account for the final wet concrete weight.
Do I need Building Regulations approval for concrete work?
Requirements depend on the project. Structural foundations, alterations and work affecting a building may need design input, Building Regulations compliance or professional inspection. Check the project requirements with the relevant local authority or a suitably qualified professional.
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disclaimer
- This calculator provides an estimate based on the volume, bag weight, bag yield, waste allowance and price entered.
- Preset bag yields are approximate. Always use the yield stated by the manufacturer for the exact product being purchased.
- Actual material use can vary because of mixing, compaction, spillage, excavation tolerances, moisture, site conditions and workmanship.
- The total weight is the combined dry bag weight and does not include added water, packaging variations or the final weight of cured concrete.
- The cost result covers bags only and excludes delivery, VAT differences where applicable, equipment, reinforcement, formwork, additives, labour and disposal.
- Check that the base, drainage, reinforcement, concrete specification, depth and curing method are suitable for the intended job.
- Concrete and cement products can cause skin burns and eye damage. Follow the product safety data, wear suitable gloves, eye protection and other required PPE, and avoid breathing dust.
- For structural work, foundations or any project where failure could cause injury or property damage, obtain advice and specifications from a suitably qualified professional.
- This content is general guidance and is not a substitute for product instructions, site assessment, structural design, Building Regulations requirements or professional advice.