Plaster Calculator for Walls and Ceilings
Enter the wall size, plaster thickness and mix to get cement bags, sand in cft and the material cost of cement plaster.
How the plaster calculation is done
Plaster is a thin layer of cement mortar, so the quantity comes from area x thickness. The steps are:
- Area. Wall length x height, minus doors and windows, times the number of faces you plaster. Enter sizes in feet; the calculator converts to sq m (1 sq ft = 0.0929 sq m).
- Wet volume. Area in sq m x thickness in metres. 12 mm is 0.012 m.
- Extra for the surface. Brick walls are not flat. Mortar fills the joints and the low spots, and some falls on the floor. The default adds 20% for this.
- Dry volume. Cement and sand shrink when mixed with water, so the dry volume is taken as 1.27 times the wet volume (after the 20%).
- Cement and sand. The dry volume is split in the mix ratio. In 1:6 there are 7 parts: 1 cement and 6 sand. Cement is changed into 50 kg bags at 0.0347 cu m per bag, and sand is shown in cu m and cft.
The cost line is cement bags (rounded up to whole bags) x the cement rate, plus sand in cft x the sand rate. Labour is not included.
Worked example (default inputs)
One face of a wall 10 ft x 10 ft, 12 mm thick, 1:6 mix, 20% extra, cement at Rs 380 a bag and sand at Rs 60 a cft.
- Area: 100 sq ft = 9.29 sq m.
- Wet volume: 9.29 x 0.012 = 0.1115 cu m.
- Dry volume: 0.1115 x 1.20 x 1.27 = 0.1699 cu m.
- Cement: 0.1699 / 7 = 0.0243 cu m = 0.70 bag (about 35 kg).
- Sand: 0.1699 x 6 / 7 = 0.1456 cu m = 5.1 cft.
- Material cost: 1 bag x Rs 380 + 5.1 cft x Rs 60 = about Rs 689.
So 12 mm plaster in 1:6 uses about 0.7 bag of cement and 5 cft of sand per 100 sq ft, or about 3.8 kg of cement per sq m. For the outside of a house, a 40 ft wall 10 ft high with 40 sq ft of windows, 20 mm thick in 1:4, needs about 5.9 bags of cement and 28.8 cft of sand.
Thickness and mix by location
| Where | Usual thickness | Usual mix |
|---|---|---|
| Ceiling | 6 mm | 1 : 3 or 1 : 4 |
| Inside walls, smooth face | 12 mm | 1 : 6 |
| Inside walls, rough brick face | 15 mm | 1 : 5 or 1 : 6 |
| Outside walls | 18 to 20 mm, often in two coats | 1 : 4 |
| Wet areas (toilets, kitchen dado base) | 12 to 15 mm | 1 : 4 |
These are common site practice, not a rule for every job. IS 1661 is the Indian code of practice for cement and cement-lime plaster, and your drawing or specification may set other values. Follow those where they exist.
Common mistakes when estimating plaster
- Forgetting the second face. An inside wall is plastered on both sides. Choose “Both faces” or the cement will run out halfway.
- Using the design thickness only. If the brickwork is out of plumb, the plaster gets thicker at the low points. 15 mm on a bad wall uses 25% more mortar than 12 mm.
- No allowance for the surface. Without the 20% extra for joints, unevenness and droppings, most estimates fall short.
- Deducting every small opening. Many contractors do not deduct small openings, because the reveals (the sides of the opening) also need plaster. Deduct doors and windows, not ventilators.
- Wet sand. Damp sand bulks up, so a cft of it has less sand. Measure dry sand if you can.
- No curing. Not a quantity mistake, but the most common reason for cracks. Keep the plaster wet for about a week.
Plaster in the full wall estimate
Plaster comes after the masonry, so estimate the wall first with the brick calculator, then add the plaster for both faces here, then the finish with the paint calculator or the tile calculator. For a whole building, the house construction cost calculator gives the budget in one place.
Plaster is often given to a gang on a per sq ft rate. Measure the work before you pay. A measurement book format keeps the record, and the free Excel templates page has registers for material and labour. In the Trackovo app, the cement and sand are requested, approved and received with a photo at site, and the measurement book and contractor settlement are kept with the site: see material management and labour management.
Common questions
How much cement is needed for 100 sq ft of plaster?
For 12 mm thick plaster in 1:6, about 0.7 bag (35 kg) of cement and 5 cft of sand, including 20% for an uneven surface. For 20 mm in 1:4 it is about 1.6 bags per 100 sq ft.
How much cement is needed per sq m of 12 mm plaster?
About 3.8 kg of cement and 0.016 cu m of sand per sq m in 1:6, with the 20% surface allowance. In 1:4 it is about 5.3 kg of cement per sq m.
Why is the dry volume factor 1.27 for plaster and 1.54 for concrete?
Concrete has three materials and large gaps between the stones, so it shrinks more when water is added. Plaster mortar has only cement and sand, so the drop in volume is smaller. Some engineers use 1.33 for mortar; you can change the factor in the script.
Which plaster mix is used for outside walls?
1:4 is the common mix for outside walls, often in two coats with a total of 18 to 20 mm. Inside walls are usually 1:6, 12 mm thick, and ceilings 1:3 or 1:4, 6 mm thick.
Should I deduct doors and windows from the plaster area?
Deduct full doors and windows, but add back the reveals if they are wide. Small openings such as ventilators are usually not deducted, because the sides of the opening take about as much plaster as the hole saves.
Request the cement and sand for each floor from these numbers, and check the receipts against the area plastered: track material and labour in Trackovo. Trackovo is a construction site management app with material, labour attendance, DPR and payments, with a 14-day free trial.
