Site Mix Adjustment Calculator
Enter the lab mix and moisture content to get real batch weights.
With the default inputs: Cement 340 kg/m³ · Sand 700 kg/m³ · Gravel 1200 kg/m³ etc.Water to add per m³ 147.0 kg/m³
Edit the inputs below to recalculate.Inputs
Result
| Sand, as batched | 721.0kg/m³ |
| Gravel, as batched | 1212.0kg/m³ |
| Water carried by aggregate | 33.0kg/m³ |
| Water to add | 147.0kg/m³ |
| Batch factor | 0.294 |
| Cement per batch | 100kg |
| Sand per batch | 212.1kg |
| Gravel per batch | 356.5kg |
| Water per batch | 43.2kg |
Wet aggregate carries water, so the extra aggregate must be offset by less added water - that is what keeps the water/binder ratio constant. Measure moisture every shift and every two hours in rain; 1% drift in sand moisture moves about 7 kg of water per cubic metre, enough to shift both slump and strength. A negative result means the lab water or the moisture reading is wrong - recheck, do not batch it.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Wet sand = dry sand × (1 + moisture) | same for gravel |
| Water added = lab water − dry sand×m − dry gravel×m | keeps w/b constant |
| Batch weight = per m³ × (cement per batch ÷ cement per m³) | batch sized to whole bags |
How to work it out
- Fix the cement per batch first, in whole bags (50 kg), because nobody weighs half a bag on site.
- Scale the design mix to that cement quantity to get the theoretical sand, stone and water per batch.
- Actual sand = theoretical sand × (1 + moisture content); the same for stone.
- Deduct the water carried in by the aggregates from the total: actual water = theoretical water − sand water − stone water. This is the step that matters.
- Verify the water-binder ratio afterwards; it must equal the design value, and adding water to make pumping easier is not allowed.
Where it goes wrong
- Using one fixed moisture content — after rain, sand can go from 3% to 8%, adding 35 kg of water per cubic metre and dropping a whole grade.
- Adding wet aggregate weights without deducting the water — the classic error: aggregates are corrected but water is not, so the ratio blows out.
- Budgeting 2.5 bags of cement per batch — only whole bags can be charged, so estimating the half bag abandons batching accuracy.
Material and cost
The price of losing control of moisture is not material cost but failed strength, rebound testing and remedial work — tens of thousands per element. A moisture test takes 5-10 min per batch and costs nothing.
On site
- Test aggregate moisture before the first batch each shift, and every 2 hours in rain. This is mandatory.
- Post the mix board at the mixer showing actual batch weights; that is what inspections check.
- Meter admixture separately, never with the cement — 0.5% dosing error shows up directly in strength.
Basis
- JGJ 55-2011 Specification for mix proportion design of ordinary concrete (site mix adjustment)
- GB 50666-2011 Code for construction of concrete structures, Section 7.2 (batching of raw materials)
- GB 50164-2011 Standard for quality control of concrete (batching tolerances: cement ±2%, aggregate ±3%, water ±2%)
FAQ
Why not just follow the lab mix and add water as needed?
How often should moisture be measured, and what about rain?
The calculated water comes out negative - what is wrong?
Cite this page
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Building Calculator. Site Mix Adjustment Calculator. 2026-10-25. https://jianzhujisuan.com/en/concrete-water-adjust