The 1.27× factor, and why it's different from concrete's 1.54×
Plaster mortar is a thinner coat with a richer cement content and finer sand than a structural concrete mix, so it bulks less when converted from wet to dry material. The standard allowance used is:
That dry volume then splits by your cement:sand ratio (commonly 1:4 or 1:6), and converts to cement bags the same way as the Concrete calculator.
Picking a thickness
Typical single-coat internal plaster runs 12-15mm. External work, or where the wall surface is uneven, often needs a two-coat build-up totalling 15-20mm. Check thickness against your local practice or drawing — thicker plaster uses proportionally more material.
Frequently asked questions
What cement:sand ratio should I use for plaster?
1:4 is common for internal work, 1:6 for a rougher scratch/base coat, and richer mixes like 1:3 are sometimes used for external or high-durability finishes. Match your local standard or specification.
Should I subtract door and window openings before entering area?
Yes — enter the net plasterable area (wall area minus doors, windows and other large openings) for an accurate material count.
Why is the dry-volume factor lower than concrete's?
Plaster is a finer, richer mortar with less coarse aggregate, so it bulks less than a structural concrete mix — 1.27 is the standard allowance used for plaster and similar thin mortar coats.