Work Out How Many Bags Of Concrete You Need For A Slab, Pad, Strip Footing Or Post Holes. Enter Your Project Measurements, Choose A Waste Allowance, Add The Actual Yield Printed On Your Concrete Bag And Get A Whole-Bag Estimate, Total Dry Mix Weight And Optional Material Cost In Seconds.
Choose A Project Shape, Enter The Measurements And Then Use The Yield From Your Exact Bagged Concrete Product. The Calculator Rounds Up To Whole Bags So The Result Is Practical For Buying Materials.
Your Result Will Appear Here After You Press Calculate Concrete Bags.
The PDF Includes The Project Type, Main Measurements, Waste Allowance, Bag Weight, Product Yield, Concrete Volume, Whole-Bag Quantity, Total Dry Mix Weight And Estimated Bag Cost.
This Example Uses A 3 m × 3 m Slab At 100 mm Thick. The Base Volume Is 0.90 m³. With A 10% Allowance It Becomes 0.99 m³. If The Exact Product Yield Is 0.010 m³ Per Bag, The Estimate Is 99 Whole Bags.
The Calculator Keeps Volume, Product Yield And Purchase Quantity Separate. That Matters Because A 20 Kg, 25 Kg Or Other Bag Weight Does Not By Itself Tell You Exactly How Much Finished Concrete The Product Will Produce.
Calculate The Geometric Volume Of The Slab, Footing, Post Holes Or Enter A Known Volume Directly.
Add An Adjustable Percentage For Site Variation, Spillage And Small Measurement Differences.
Divide Required Concrete Volume By The Actual Yield Per Bag And Round Up To Whole Bags.
Multiply The Number Of Bags By Your Selected Bag Weight To See The Total Dry Product Weight.
Enter Your Own Current Price Per Bag To Build A Simple Concrete-Material Budget.
Save The Main Measurements And Bag Estimate In A Clean PDF Generated By ConcreteCreek.com.
A Concrete Bag Calculator answers a simple buying question: once you know the concrete volume of a project, how many bags of premixed concrete are required? The important detail is that the calculation should be based on yield per bag, not bag weight alone. Two products can have the same nominal bag weight yet use different mix proportions, aggregates or instructions, so the volume of mixed concrete produced by one bag can differ.
The safest estimating method is therefore to calculate the project volume first, add any reasonable allowance you choose for real-site variation, and divide that adjusted volume by the yield stated on the exact bag or manufacturer product data sheet. The calculator above follows that method and then rounds the answer up to a whole bag because concrete is normally purchased in complete bags.
Bags Required = Concrete Volume With Allowance ÷ Yield Per Bag
For a rectangular slab or pad, multiply length by width by depth. Keep all three measurements in metres before multiplying. If the slab thickness is entered in millimetres, divide by 1000 first. For example, 100 mm becomes 0.10 m, 125 mm becomes 0.125 m and 150 mm becomes 0.15 m.
Volume (m³) = Length (m) × Width (m) × Depth (m)
If you need a broader volume-only calculation before converting the result into bags, use the internal Concrete Calculator Australia. If you already work in imperial volume, the Cubic Feet To Concrete Bags guide can help you move between the units used on different projects.
The exact geometric volume assumes every edge, base and excavation matches the measured dimensions perfectly. Real projects are often less tidy. Formwork can bow, hand-dug holes can be wider at one point, a prepared base may have low spots and a small amount of material can remain in the mixer or be lost during placement. For that reason, many people choose to add a modest planning allowance.
There is no universal percentage that is correct for every job. A neat formed pad on a accurately prepared base may need very little extra. Irregular excavation or multiple post holes can be harder to measure. The calculator lets you choose the percentage rather than forcing a fixed rule.
This is the most important product-specific number. The yield tells you the approximate volume of mixed concrete produced when one bag is prepared according to the manufacturer instructions. Look for the yield on the bag, technical data sheet or official product information. Enter that value in cubic metres per bag.
For Australian packaged-concrete product information, you can check the manufacturer documentation for the exact mix. The master reference page also points users to Boral packaged concrete information as one example of a manufacturer source.
Once the volume with your chosen allowance is known, divide it by the product yield. If the mathematical answer is 78.2 bags, the purchase estimate becomes 79 bags because a fraction of a bag is not a reliable buying unit. The calculator automatically uses a whole-bag ceiling rather than standard rounding.
| Example Yield Per Bag | Bags For 0.50 m³ | Bags For 1.00 m³ | Bags For 2.00 m³ |
|---|---|---|---|
| 0.006 m³ | 84 | 167 | 334 |
| 0.008 m³ | 63 | 125 | 250 |
| 0.010 m³ | 50 | 100 | 200 |
| 0.012 m³ | 42 | 84 | 167 |
| 0.015 m³ | 34 | 67 | 134 |
The table above is deliberately organised by example yield, not by bag weight. It shows why the yield input matters so much. If your product produces 0.010 m³ per bag, one cubic metre corresponds mathematically to 100 bags. If the actual product yield is smaller, more bags are needed; if the yield is larger, fewer bags are needed.
A 20 kg concrete bag calculator should still ask for yield. The “20 kg” label tells you the dry packaged weight, while yield tells you the finished concrete volume produced after correct mixing. On this page, the default example uses a 20 kg bag and an editable 0.010 m³ yield, but those are only starting values so you can see how the tool works. Replace them with the figures for the actual product you plan to use.
If you are comparing bag quantities with other units, the internal Concrete Bags To Cubic Metres page can help explain the reverse calculation. For imperial planning, you may also find Concrete Bags To Cubic Feet useful.
Slabs are straightforward when the shape is rectangular. Measure the inside dimensions of the finished formwork where possible. Multiply the length and width in metres by the thickness in metres. A 3 m × 3 m slab that is 100 mm thick has a base volume of 0.90 m³. If you choose a 10% allowance, the planning volume becomes 0.99 m³.
If your product yield is 0.010 m³ per bag, 0.99 ÷ 0.010 = 99 bags. If the same project used a product with a different stated yield, the bag count would change even though the slab dimensions stayed exactly the same.
For a straight strip footing, multiply the total footing length by its width and depth. Width and depth are often specified in millimetres, so convert them to metres. A 10 m long footing that is 300 mm wide and 300 mm deep has a base volume of 10 × 0.30 × 0.30 = 0.90 m³ before any extra allowance.
Footings can be less uniform than formed slabs because excavation may widen or deepen in places. Recheck the trench after digging rather than relying only on the planned dimensions. Structural footing dimensions should come from the plans, engineer or other appropriate project requirements; a bag calculator only estimates material quantity.
A round post hole is treated as a cylinder. Convert the diameter and depth to metres, divide the diameter by two to get the radius, and then use π × radius² × depth. Multiply by the number of identical holes. The calculator also offers a simple post-displacement percentage because the inserted post occupies some of the hole volume.
Hole Volume = π × Radius² × Depth × Number Of Holes
For more precise work, calculate the actual displacement from the real post cross-section rather than relying on a general percentage. The percentage option is intended as a planning shortcut, not a structural design method.
Select Known Volume in the calculator and enter the cubic metres directly. This is useful if an engineer, builder, another calculator or a supplier worksheet has already provided the concrete quantity and you only want to convert that number into bags.
Once the whole-bag quantity is known, total dry mix weight is simply the number of bags multiplied by the bag weight. If a project needs 99 bags and each bag weighs 20 kg, the packaged dry material weighs 1,980 kg in total. This can be useful when thinking about transport, storage and manual handling.
Dry Mix Weight = Whole Bags Required × Bag Weight
A high bag count also signals a handling issue. Do not treat the weight figure as a lifting recommendation. Plan delivery, storage and handling appropriately and follow workplace or manufacturer safety guidance.
Enter the current price per bag and the calculator multiplies it by the whole-bag quantity. This gives a material-only estimate for the packaged concrete. It does not include delivery fees, mixer hire, tools, reinforcement, formwork, excavation, labour, finishing, curing materials or disposal.
Prices can change by retailer, location, product type and order quantity, so using an editable price field is more useful than publishing one fixed number. For a meaningful project budget, update the field with the current local price for the exact product.
Bagged concrete is convenient for small repairs, fence posts, isolated pads and locations where a ready-mix truck is impractical. As project volume increases, however, the number of bags can rise quickly. Hundreds of bags require substantial transport, storage, opening, mixing and consistent water control.
For larger pours, compare the bag estimate with a delivered ready-mix quote. Use the Concrete Calculator Australia to verify cubic metres and then ask a local supplier about minimum loads, delivery, short-load charges, site access and waiting time.
This page estimates concrete quantity and bag count. It does not select slab thickness, footing dimensions, reinforcement, concrete strength, exposure classification, joints, slump, curing method or structural design. Those requirements must come from the appropriate drawings, specifications, manufacturer instructions, builder, engineer or other qualified project source where applicable.
Keep Project Volume, Waste, Yield And Bag Weight Separate. Mixing Them Together Is One Of The Main Reasons Bag Estimates Go Wrong.
The Concrete Space To Fill Before Converting The Requirement Into Bags.
An Adjustable Buffer For Real-Site Differences, Not A Fixed Rule For Every Job.
The Product-Specific Number Used To Convert Required Concrete Into Whole Bags.
Useful For Total Dry Mix Weight, But Not A Substitute For Manufacturer-Stated Yield.
These Examples Use A 10% Allowance And An Example Yield Of 0.010 m³ Per Bag Only To Demonstrate The Maths. Replace The Yield With The Value For Your Exact Product.
| Project Example | Dimensions | Base Volume | With 10% Allowance | Bags At 0.010 m³ Yield |
|---|---|---|---|---|
| Small Slab | 3 m × 3 m × 100 mm | 0.90 m³ | 0.99 m³ | 99 Bags |
| Garden Pad | 2 m × 2 m × 100 mm | 0.40 m³ | 0.44 m³ | 44 Bags |
| Concrete Path | 6 m × 1 m × 100 mm | 0.60 m³ | 0.66 m³ | 66 Bags |
| Strip Footing | 10 m × 300 mm × 300 mm | 0.90 m³ | 0.99 m³ | 99 Bags |
| Known Volume | 1.00 m³ Before Allowance | 1.00 m³ | 1.10 m³ | 110 Bags |
The Price-Per-Bag Field Gives A Material Estimate, But The Full Job Cost Can Include Much More Than The Packaged Concrete Itself.
Straightforward Answers About 20 Kg Bags, Product Yield, Concrete Volume, Waste, Cost And Whole-Bag Rounding.
Calculate The Required Concrete Volume, Add Your Chosen Allowance, Divide By The Yield Stated For The Exact Product And Round Up To A Whole Bag.
The Exact Number Depends On The Manufacturer-Stated Yield Per 20 Kg Bag. Divide 1 m³ By That Yield And Round Up. Do Not Assume Every 20 Kg Product Has The Same Yield.
No. Bag Weight Is The Dry Packaged Mass. Yield Is The Approximate Mixed Concrete Volume Produced Per Bag. The Bag Count Calculation Needs Yield.
Multiply Slab Length By Width By Depth In Metres To Get Cubic Metres, Add Any Chosen Allowance, Then Divide By The Product Yield Per Bag And Round Up.
Find The Cylindrical Volume Of One Hole Using π × Radius² × Depth, Multiply By The Number Of Holes, Allow For Post Displacement If Appropriate, Then Convert The Required Volume Into Bags Using Product Yield.
A Small Allowance Can Help Cover Uneven Excavation, Formwork Differences, Spillage And Measurement Variation. The Right Percentage Depends On The Actual Project.
Concrete Bags Are Purchased As Whole Units. If The Calculation Produces A Fractional Bag Requirement, Rounding Down Could Leave The Project Short Of Material.
Yes. Choose Known Volume And Enter The Base m³ Directly. The Calculator Can Then Add Waste And Convert The Result Into Bags.
No. It Multiplies Whole Bags By The Price Per Bag You Enter. Delivery, tools, mixer hire, reinforcement, formwork, labour and other project costs are separate.
Yes. Change The Bag Weight And, More Importantly, Enter The Actual Yield For That Exact Product. The Calculator Is Not Limited To 20 Kg Bags.
As Project Volume Increases, The Bag Count And Mixing Work Can Become Very Large. Compare A High Bag Estimate With A Delivered Ready-Mix Quote And Consider Access, labour, mixing capacity and pour continuity.
The PDF Includes Project Type, Measurements, Waste Allowance, Bag Weight, Yield, Concrete Volume, Whole-Bag Quantity, Total Dry Mix Weight And Estimated Bag Cost.
Use Current Product Data Sheets, Manufacturer Instructions And Australian Safety Guidance Alongside Any Online Concrete Bag Estimate.
Australian Industry Information And Technical Concrete Resources.
Visit CCAANational Information About Silica Hazards And Safe Work Requirements.
Read Silica GuidanceExample Manufacturer Information For Packaged General Purpose Concrete.
View Product InformationConfirm Yield, Water Addition, Intended Uses, Mixing Instructions, Safety Information And Storage Requirements Before Purchase.