Insulation Calculator: How Much Insulation Do I Need?
Enter the wall or ceiling area and how it is framed. The number you need is not the gross area — studs, plates and headers take up a real share of any framed wall and do not get insulated, so buying the gross area over-orders.
Surfaces to insulate
Measure the full wall or ceiling — the framing allowance is taken off for you.
The framing allowance is an estimate, not a measurement: real values range from about 15% to 25% depending on spacing, corners, headers and how many openings a wall has. Buy to the cavity area and expect to cut pieces to fit.
How much insulation do I need?
Measure the wall or ceiling, then take off the share occupied by framing. What is left is the cavity area — the actual gaps between the studs or joists that insulation goes into.
Insulation performs on contact. A batt that is compressed, cut short or stuffed around a wire loses much of its value, so it is worth buying enough to cut pieces properly rather than stretching what you have.
How much of a wall is framing?
| Spacing | Typical framing allowance |
|---|---|
| 16 in on centre | 15% |
| 24 in on centre | 10% |
These are starting points. A wall with several windows, a door and two corners is closer to 25% framing than 15%.
An open joist bay in an attic has effectively no framing to deduct, which is why zero is a legitimate value here rather than an error.
A note on R-value
This calculator works out how much insulation to buy, not how well it will perform. R-value depends on the product, the depth of the cavity, how well it is fitted and what else is in the wall assembly — none of which a quantity estimate can speak to.
Pick the R-value your climate and local code call for, then use this to work out how many packs of it you need. Anything beyond that is a question for a professional, not a calculator.
How to measure
Add each as its own area. They are summed before the framing allowance is applied.
The allowance takes the framing off for you — deducting it twice under-orders.
Batt and roll coverage varies widely by R-value and width; it is on the label.
Common mistakes
- Buying the gross wall area and ending up with unused packs.
- Using a 16 in allowance on a wall that is mostly window.
- Compressing batts to make them fit, which throws away the performance you paid for.
- Treating a quantity estimate as a thermal specification.
Boarding over it next?
Work out the sheets, screws and compound for the same walls.
Open the Drywall Calculator →