Cement Bag Calculator
Total cement, exact bags and whole bags to order.
Quick answer
With the default inputs: Volume 1 m³ · Cement per m³ 320 kg/m³ · Bag weight 50 etc.Whole bags to order 7 bags
Edit the inputs below to recalculate.Inputs
Result
Whole bags to order
7bags
| Cement before waste | 320.0kg |
| Including waste | 329.6kg |
| Exact bags | 6.59bags |
| Surplus | 20.4kg |
| In tonnes | 0.330t |
| Mass of bags ordered | 350kg |
| At 20 bags per tonne | 6.59bags |
| Mortar per bag | 0.156m³ |
Rules of thumb for cement content: C25 takes about 320 kg/m³, C30 377 and C40 440; M10 mortar about 300 kg/m³ and a 1:2 cement mortar 550. Bags must be bought whole; bulk is cheaper, so switch to a silo once a site needs more than 50 t.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Total cement | volume × cement per m³ |
| With waste | total × (1 + waste%) |
| Exact bags | with waste ÷ bag weight |
| Whole bags | ceil(exact bags) |
| Surplus | bags × weight − with waste |
| Tonnes | with waste ÷ 1000 |
1 m³ of C25 at 320 kg/m³ with 50 kg bags and 3% waste: 320.0 kg of cement, 329.6 kg with waste, 6.59 bags exactly — order 7 for a 20.4 kg surplus.
How to work it out
- Establish the cement content per cubic metre from the mix design, never from habit: C30 concrete takes 320-400 kg/m³, a 1:3 mortar about 400 kg/m³.
- Total cement = quantity × content per m³ × (1 + waste). Waste is 3-5%, and bagged cement also loses torn bags and damp lumps.
- Exact bags = total ÷ bag weight. The decimal matters for costing.
- Purchase bags = exact bags rounded up. On a large job the rounding is nothing; on a small one, one extra bag is 20%.
- Bagged cement has a shelf life of about three months, less in damp conditions, so buy no more than a month's usage.
Where it goes wrong
- Estimating cement by volume — the rule of seven bags per cubic metre is wrong for C20 (about 5.5) and wrong for C40 (about 9); a grade change breaks it.
- Ignoring waste — torn bags, lumps, spillage and hardened residue all count, so 3-5% is the floor.
- Mixing bagged and bulk costing — bulk is priced per tonne and bagged per bag with packaging included, a 10-20% difference.
- Stockpiling beyond shelf life — strength drops 10-20% in three months and over 30% in six, so the saving is wiped out.
Material and cost
Bagged P.O 42.5 380-520 CNY/t (19-26 CNY per 50 kg bag), bulk 350-480. Slag cement is 30-60 cheaper, rapid-hardening 50-100 dearer.
On site
- Bags are declared at 50 kg with a ±1% tolerance (49.5-50.5 kg), so spot-check by weight on bulk purchases.
- Lumpy damp cement must not go into structural work; lightly lumpy material can be sieved for bedding mortar at reduced strength.
- Never mix cement types — Portland, ordinary Portland, slag and fly ash blends have different setting behaviour.
Basis
- GB 175-2023 Common Portland cement
- JGJ 55-2011 Specification for mix proportion design of ordinary concrete
- GB 50204-2015 Code for acceptance of concrete structure construction, Chapter 7
FAQ
Why 3% for cement waste?
Bagged cement loses 1-3% in opening, tipping and mixer residue, while silo discharge loses 0.5-1%. Admixtures and fly ash follow the same logic. Three percent is the norm; on small jobs with hand mixing and few bags, 5% is safer because waste scales differently between 20 bags and 200.
Can damp, caked cement still be used?
Lightly caked cement that crumbles to powder can be screened and used at a lower grade after strength testing; lumps that will not crush have already hydrated with over 20% strength loss and must be scrapped. Beyond three months in store, strength drops 10-20%, and after six months it must be retested. Store it covered, off the ground, 300 mm clear of walls and no more than ten bags high.
Why not simply add more cement?
Because more cement is not more strength. Excess cement raises heat of hydration, increases shrinkage and cracking, and reduces durability. Strength comes from a low water-binder ratio, not from extra cement. Codes set both a minimum, for durability, and an effective upper limit, for heat — C30 usually sits between 320 and 400 kg/m³.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Cement Bag Calculator. 2026-10-25. https://jianzhujisuan.com/en/cement-bag
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.