Gypsum Calculator
Estimate the dry gypsum powder a coating or a casting needs, the water for mixing and the number of bags to buy.
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Formula and method
Net area times layer thickness times the consumption rate gives the dry mass. Multiplying that mass by the water-to-gypsum ratio gives the mixing water, and dividing by the bag weight gives the number of bags, rounded up.
Dry mass first, water second
Gypsum is bought as powder and used as a slurry, so the useful number is the dry mass. Area × thickness × consumption gives the powder, and the mixing water is that mass times the water ratio, usually 0.6 to 0.7 litres per kilogram. For 14.5 m² at 10 mm and 0.9 kg/m² per mm that is 130 kg of powder and about 85 litres of water.
The water figure is not decoration: it decides how many buckets and how much time the batch takes, and it is the difference between a mix that spreads and one that sets in the bucket.
Working time per batch, not per wall
Gypsum sets in 20 to 40 minutes depending on the product and the water temperature, and it does not stop setting because you have not finished. A batch is sized to what two people can place in twenty minutes, typically one or two bags, not to the whole wall.
Cold water lengthens the set, warm water shortens it. Never try to retemper a mix that has started to stiffen: adding water to set gypsum destroys the crystal structure and the patch will be soft and powdery.
Thickness, waste and the expansion
Gypsum expands slightly as it sets, which is why it is good for filling deep pockets, but it also means a thick pour in a confined space can crack from its own heat. Deep fills are built up in layers rather than poured in one go.
Add 10 to 15 % for waste: mixing losses, the residue in buckets and the material left in the hose. The reserve is small in money and large in schedule, because running out mid-wall means a visible cold joint.
How to measure the inputs
- Measure each wall length and height, multiply by the number of identical walls and subtract openings.
- Estimate the average layer thickness — for levelling, use the deepest point plus 2–3 mm.
- Read consumption and bag weight from the packaging and the water ratio from the mixing instructions.
Worked example
Wall length 16.4 ft; Wall height 9.84 ft; Wall count 1; Openings area (optional) 5.38 ft²; Layer thickness 0.39 in; Consumption rate 0.9 kg/m² per mm; Water per 1 kg 0.65 l/kg; Package weight 66.14 lb; Waste 10 % → Gypsum bags: 5
Typical gypsum figures for the calculation
| Property | Typical value | What it changes |
|---|---|---|
| Mixing water per 1 kg of powder | 0.60–0.70 l | less water: stronger and faster setting |
| Start of setting | 6–10 min | how large one batch can be |
| End of setting | 20–30 min | no water may be added after this point |
| Consumption at a 10 mm layer | 8–10 kg/m² | mass and bag count for a finishing coat |
Practical tips
- Add gypsum to water, not water to gypsum — it wets evenly and forms fewer lumps.
- Never add water to a mix that has started to set: the strength is lost for good.
- Check the date on the bag; damp powder sets in lumps and behaves unpredictably.
Source
- EN 13279-1 — Gypsum binders and gypsum plasters
Checked and updated 21 October 2026