How to Calculate How Much Carpet You Need
I built this carpet calculator to give you an accurate material estimate before you walk into the flooring store. Getting the right square footage upfront prevents the two most common carpet shopping mistakes: buying too little and ending up with a visible seam in an awkward spot, or buying so much that you waste hundreds of dollars on leftover rolls you'll never use.
The basic calculation is simple: multiply the room length by the room width to get the square footage. For rectangular rooms that's all there is to it. For L-shaped or irregular rooms, break the space into rectangles, calculate each one separately, and add them together. Then add your waste factor before placing the order.
Don't Forget the Waste Factor
Carpet comes on rolls that are typically 12 feet wide (and sometimes 15 feet wide for commercial grades). If your room is 14 feet wide, you'll need two strips — and the offcut from the second strip is largely wasted. The waste factor you need depends on your room shape and pile direction:
- Simple rectangular rooms: 5–10% waste is usually enough.
- Rooms with angled walls, bay windows, or alcoves: budget 15% waste.
- Cut-pile carpets with a directional nap: all strips must run the same way, which increases offcuts.
- Patterned carpets with a repeat: add the repeat length multiplied by the number of seams to your total.
Measuring Your Room Correctly
Measure at the widest point of each dimension, including any alcoves, closets, or under-door thresholds that will be carpeted. It's cheaper to have a little extra than to come up short when you're fitting around a built-in wardrobe. I always tell people to measure twice and order once — returning carpet to a supplier is painful, but running short mid-installation is worse.
Carpet is typically priced per square yard in the US and per square metre in most other countries. If your supplier quotes per square yard, divide your square footage by 9. This calculator handles the conversion automatically, so you can work in whichever unit feels natural and get results in both.
Frequently Asked Questions
How much extra carpet should I order?
For most rooms I recommend ordering at least 10% extra. This covers the standard installation waste from trimming edges, plus gives you offcuts to keep for future repairs. If a pet or furniture leg damages a patch of carpet years later, having a matching remnant from the same dye lot is invaluable — you won't be able to buy it again.
Can I install carpet over existing carpet?
Technically yes, but professional installers strongly advise against it. Layering carpet raises the floor height, which can create tripping hazards at doorways and voids the warranty on most carpet products. It also traps moisture between layers. The right approach is to remove the old carpet, inspect the subfloor for damage or squeaks, and start fresh with proper underlay beneath the new carpet.
What underlay thickness should I use?
A good underlay makes a significant difference to how carpet feels underfoot and how long it lasts. For residential use, 8–10 mm thick underlay with a density of 130 kg/m³ is the standard recommendation. Thicker underlay (10–12 mm) feels more luxurious but can cause issues with door clearance. For stairs, use a thinner, firmer underlay (around 6 mm) to prevent the carpet from bunching on the nosing.
