Imagine this: You’re excited to refresh your home with a new coat of paint. You head to the store, pick a colour, and buy what you think is enough paint—only to run out halfway through the job. Now, you’re stuck with mismatched batches or an unfinished room. Frustrating, right?
Knowing how much paint to paint a house interior is essential to avoid overspending, unnecessary trips to the store, or leaving a project incomplete. But calculating paint isn’t just about guesswork. Factors like room size, surface type, and the number of coats needed all play a role in getting it right.
As professional painters in the Southern Highlands, we understand how overwhelming this can feel. That’s why after reading this guide, you will learn everything from simple calculations to pro tips—so you can paint with confidence and precision.
Factors That Affect Paint Consumption
Before you rush out to buy paint, it’s important to understand what determines how much you’ll actually need. Here are the key factors that can impact paint usage:
1. Room Size and Surface Area
The bigger the space, the more paint you’ll need—simple, right? But it’s not just about the room’s square footage. You also need to consider:
- Wall height – Taller walls mean more surface area to cover.
- Ceilings – Are you painting the ceiling too? This adds to the total paint requirement.
- Windows and doors – These reduce the total wall space that requires paint.
2. Number of Coats Required
One coat isn’t always enough. You may need additional coats in these situations:
- Dramatic colour changes – Going from dark to light (or vice versa) often needs a primer and multiple coats.
- Poor wall condition – Old, patchy, or stained walls may absorb more paint, requiring extra coverage.
- Low-quality paint – Cheaper paints often require more coats to achieve an even finish.
3. Type and Finish of Paint
Different types of paint have different coverage rates:
- Matte or flat paint absorbs more and may require additional coats.
- Satin and semi-gloss paints offer better coverage, sometimes needing fewer coats.
- Ceiling paints tend to be thicker, covering in one or two coats.
4. Surface Condition and Preparation
The condition of your walls plays a huge role in how much paint you’ll use.
- Bare plaster or unpainted drywall absorbs more paint, requiring a primer coat.
- Textured walls (like brick or render) have more surface area than smooth walls, meaning more paint is needed.
- Previously painted surfaces in good condition usually need less paint.
By considering these factors, you can get a more accurate estimate of how much paint to buy. Next, we’ll walk through a step-by-step method to calculate exactly how much you need.
How to Calculate Paint Needed (Step-by-Step Guide)
Now that you understand what affects paint usage, let’s break down how to accurately calculate how much paint to paint a house interior. Follow these simple steps:
Step 1: Measure the Wall and Ceiling Areas
You need to find the total square metres of the walls and ceilings you’re painting. Here’s how:
- Measure the width and height of each wall.
- Multiply width × height to get the wall area in square metres.
- Repeat for each wall in the room and add up the total.
- If painting the ceiling, measure the length × width for its area.
Example Calculation:
A room with four walls, each 4m wide and 2.5m high:
- 4m × 2.5m = 10m² (per wall)
- 10m² × 4 walls = 40m² total wall area
For a ceiling that is 4m × 4m = 16m²
Step 2: Subtract Windows and Doors
Since doors and windows don’t need paint, subtract their area:
- Standard door: 2m²
- Standard window: 1.5m²
Example:
- If a room has one door (2m²) and two windows (3m² total):
- Total wall area: 40m²
- Subtract doors & windows: 40m² – 5m² = 35m²
Step 3: Check Paint Coverage per Litre
Most paint cans specify coverage per litre on the label. On average:
- One litre of paint covers ~12m² per coat
- For two coats, divide by 6m² per litre
Example:
If your total paintable area is 35m² and you need two coats:
- 35m² ÷ 6m² (per litre) = 5.8 litres of paint
- Round up to 6 litres to be safe
Step 4: Adjust for Extra Factors
- Rough or textured surfaces? Add 10-20% more paint
- Dramatic colour changes? You may need an extra coat
- Touch-ups & future use? Buy an extra litre for later
By following this step-by-step method, you can confidently estimate how much paint you need without overbuying or running short.
How to Avoid Buying Too Much or Too Little Paint
Getting the right amount of paint can save you time, money, and hassle. Here’s how to avoid overbuying or running short.
1. Plan for Touch-Ups and Leftover Paint
- Always buy 10–15% extra paint to cover touch-ups or mistakes.
- Store any leftover paint properly—it’s great for future touch-ups.
- If using multiple batches, mix them together to ensure colour consistency.
2. Use a Paint Calculator
Online paint calculators can provide quick estimates. Just input:
- Room dimensions (length, width, height)
- Number of coats needed
- Type of surface (smooth or textured)
3. Consider Paint Efficiency Tips
- Primer first – If your walls are very dark or patchy, a primer can reduce the amount of paint needed.
- Use high-quality paint – Cheap paints often require more coats, costing more in the long run.
- Apply paint evenly – Thick, uneven layers use more paint than necessary.
By planning carefully, you can buy just the right amount of paint without waste.
Final Tips for a Smooth Painting Project
Now that you know how much paint to buy and whether to DIY or hire a professional, here are some final tips to ensure a successful project.
1 Choose Quality Paint
- Cheaper paints often require more coats, making them more expensive in the long run.
- Invest in high-quality paint for better coverage, durability, and easier cleaning.
- Consider washable paints for high-traffic areas like hallways and children’s rooms.
2 Store Leftover Paint Properly
- Keep leftover paint for touch-ups by sealing the can tightly and storing it in a cool, dry place.
- Label the can with the room name and date for future reference.
- If you no longer need the paint, check for local recycling programs instead of throwing it away.
3 Ensure a Flawless Finish
- Prep your walls properly by cleaning, patching holes, and using primer if needed.
- Use the right tools—rollers for large areas, brushes for edges and trims.
- Apply paint evenly in thin coats, letting each coat dry before adding another.
- Keep windows open for ventilation and faster drying.
Final Thoughts
Knowing how much paint to paint a house interior helps you save time, money, and effort. By considering factors like room size, number of coats, and paint quality, you can plan your project efficiently.
Whether you decide to DIY or hire a professional, careful preparation is key to achieving a long-lasting, beautiful finish. If you’re in the Southern Highlands and want expert advice or professional painting services, contact Southern Highlands Painting and Decorating for a flawless result!