# Paint Calculator: How Many Litres of Paint for a Room

> A paint quantity calculation works out the area to be painted, the walls less doors and windows plus the ceiling if included, multiplies it by the number of coats, and divides by the paint's coverage for each litre. A small allowance covers touch-ups and wastage.

## What it calculates

The calculator gives the quantity of **finish paint** and **primer** for a room, in litres,
along with:

- the **surface to paint**, walls less doors and windows, plus the ceiling if chosen;
- the **area of openings** left out;
- a **breakdown** of how the surface area was built up.

## Inputs

**Room length**, **width** and **wall height** are the inside dimensions in feet. Use the
height to the false ceiling where there is one, since the wall above it is not painted.

**Paint** chooses walls only, or walls and ceiling.

**Doors** and **windows** are counts; their sizes are in the assumptions panel.

**Finish coats** and **finish paint coverage** come from the paint specification and the
product datasheet.

The **assumptions** panel holds primer coats and coverage, the door and window sizes, and a
touch-up and wastage allowance.

## Method

1. Gross wall area = 2 × (length + width) × height.
2. Openings = doors × door width × door height + windows × window width × window height.
3. Surface to paint = gross wall area − openings, plus length × width if the ceiling is
   included.
4. Finish paint = surface × coats ÷ coverage × (1 + allowance).
5. Primer = surface × primer coats ÷ primer coverage × (1 + allowance).

## Assumptions

| Assumption | Default | Notes |
| --- | --- | --- |
| Finish coats | 2 | Over one coat of primer |
| Finish paint coverage | 120 sq ft a litre for each coat | Replace with the datasheet figure |
| Primer coats | 1 | 0 if the wall is already primed |
| Primer coverage | 100 sq ft a litre for each coat | Replace with the datasheet figure |
| Door | 3 × 7 ft | Change for wider or double doors |
| Window | 4 × 4 ft | Change to your window sizes |
| Touch-up and wastage | 10% | Rollers, trays, cutting-in and touch-ups |

Coverage is the input that matters most. Datasheets give it for a smooth surface under good
conditions; site coverage on new or textured walls is usually lower.

## Limitations

- **Rectangular rooms only.** Add walls of other shapes separately.
- **Openings are averaged.** Rooms with full-height glazing or many doors should use their
  real opening sizes.
- **No putty, texture or special finishes.** Those have their own coverage and process.
- **Paint is sold in fixed pack sizes.** Round the litres up to the packs available, and buy
  the whole quantity from one batch so the shade matches.
- **It does not choose the paint.** Washable, low-VOC, anti-bacterial and other finishes
  are a specification decision for the space.

## Worked example

A 20 ft × 15 ft room with 10 ft walls, one door and two windows, walls only, two finish coats
at 120 sq ft a litre and one primer coat at 100 sq ft a litre:

| Step | Calculation | Result |
| --- | --- | --- |
| Gross wall area | 2 × (20 + 15) × 10 | 700 sq ft |
| Openings | 1 × 3 × 7 + 2 × 4 × 4 | 53 sq ft |
| Surface to paint | 700 − 53 | 647 sq ft (60.1 m²) |
| Finish paint | 647 × 2 / 120 × 1.10 | 11.9 L |
| Primer | 647 × 1 / 100 × 1.10 | 7.1 L |

The room needs about **12 litres of finish paint and 7 litres of primer**. Painting the
ceiling as well adds 300 sq ft, taking the totals to about 17.4 litres of paint and 10.4
litres of primer.

## How engineers use it

On a fit-out, painting quantities are measured from the drawings in square metres for each
finish and each surface type, and the specification sets the full system: surface
preparation, putty, primer and the number of finish coats. The quantity of material then
follows from the manufacturer's coverage for that system. A calculation like this one is the
quick check that a supplier's or painter's material estimate is in the right range.

## When a professional design must replace it

Use the calculator to estimate how much paint to buy for a room or to check an estimate.
Replace it with a measured take-off and a written paint specification for a whole office or
building, where surfaces, finishes and colours vary from room to room and where the
preparation work is a large part of the job.

## Frequently asked questions

### How many coats of paint does a wall need?

Two finish coats over a primer is the usual specification for interior emulsion on new or repaired walls. A strong colour change, a dark colour or a porous surface may need a third. The paint manufacturer's system specification for the product is the reference.

### What coverage should I use?

Use the figure on the product's datasheet, for one coat on your kind of surface. Coverage falls on rough, porous or textured walls and rises on smooth, sealed ones, so the same paint can cover noticeably less on new plaster than on a previously painted wall. The calculator starts at 120 sq ft a litre for each coat; change it to the datasheet figure.

### Do I need primer?

New plaster, putty, repaired patches and walls changing from a dark to a light colour normally need a primer coat so the finish coats cover evenly and adhere properly. A sound, previously painted wall in a similar colour may not. Set primer coats to zero if none is needed.

### Is putty included?

No. Wall putty is applied before primer to level the surface, and its quantity depends on how uneven the wall is and how many coats are needed. Estimate it from the putty manufacturer's coverage figures.

### Should I include the ceiling?

Only if it is being painted in the same exercise. Ceilings often use a different paint and colour, and in offices are often a false ceiling with its own finish. Choose walls and ceiling only when the same product goes on both.

## Sources

- [IS 15489](https://standards.bis.gov.in/website/published-standards/department-wise) — Paint, plastic emulsion: specification

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Source: https://hagerstone.com/calculators/paint-quantity-calculator