Insulation Calculator: How Much Insulation Do I Need?
Quick answer: for a 1,000 sq ft attic insulated to R-38 with blown-in cellulose and no existing insulation, this insulation calculator gives 10.3 in of depth, about 856 cu ft of material and 80 bags (30 lb bags). Insulation is not bought by the R-value alone: what you order is depth and coverage, and both follow from one simple relationship — R-value equals depth multiplied by the R-per-inch of the material you choose. Enter your space, your target and your material below, and the calculator does that maths for you, in imperial (sq ft / inches) or metric (m² / cm).
Quick answer by target R-value
The table shows the depth of blown-in cellulose (R-3.7 per inch) needed to reach common targets from empty, with no existing insulation. Choose a different material in the calculator and the depth changes, because every material earns its R-value at a different rate per inch.
| Target R-value | Depth of blown-in cellulose |
|---|---|
| R-13 (wall) | 3.5 in |
| R-21 (2×6 wall) | 5.7 in |
| R-30 (floor) | 8.1 in |
| R-38 (attic, Zones 1–2) | 10.3 in |
| R-49 (attic, Zones 3–4) | 13.2 in |
| R-60 (attic, Zones 5–8) | 16.2 in |
Check a row by hand: R-38 ÷ 3.7 per inch = 10.27 in, which rounds to 10.3 in. That one division is the heart of everything on this page — the calculator simply adds area, existing insulation and bag counts around it.
Insulation Calculator
Enter your space, target R-value and material. Results update as you type, or press Calculate.
Enter your measurements, then press Calculate.
Bag counts are shown only for the two blown-in materials and use a 30 lb bag. Follow your product’s coverage chart — densities vary by brand.
How the R-value maths works
R-value measures how well a layer of material resists heat flow. The higher the R-value, the slower heat moves through it. For any single material the relationship is exact and reassuringly simple: R = depth × R-per-inch. A material rated at R-3.7 per inch gives you R-7.4 at two inches, R-18.5 at five inches and R-37 at ten inches. Turn the formula around and you get the one this calculator uses most: depth needed = R-value you still need ÷ R-per-inch.
Two adjustments sit on top of that division. First, insulation already in place counts. If an attic floor already holds R-19 and your target is R-49, you do not need R-49 of new material — you need R-30 more, because R-values of layers in the same assembly simply add together. Second, depth becomes volume once you multiply by area: a 1,000 sq ft attic at 10.3 in deep holds 1,000 × (10.3 ÷ 12) ≈ 856 cu ft of material. Volume is what turns into bags for blown-in products, because bags are filled by weight and installed at a known density.
Worked example, start to finish. Target R-49, existing R-19, blown-in cellulose at R-3.7 per inch. R to add = 49 − 19 = 30. Depth to add = 30 ÷ 3.7 = 8.1 in. That is the third test this calculator was built against, and you can reproduce it above in seconds: set the target to 49, the existing R-value to 19 and the material to blown-in cellulose. Notice how much less material a top-up needs than a from-empty job — the first inches are already there, doing their share of the work.
R-value per inch by material
Materials earn R-value at very different rates, which is why the same target can mean 5 in of one product and 15 in of another. The six values below are the ones the calculator uses. They are typical published figures for each product family; a specific brand can sit a little above or below them, so treat the printed label on your product as the final word.
| Material | R-value per inch | Depth for R-38 |
|---|---|---|
| Fiberglass batts | 3.2 | 11.9 in |
| Blown-in fiberglass | 2.7 | 14.1 in |
| Blown-in cellulose | 3.7 | 10.3 in |
| Mineral wool batts | 4.2 | 9.0 in |
| Open-cell spray foam | 3.7 | 10.3 in |
| Closed-cell spray foam | 6.5 | 5.8 in |
The pattern matters when space is limited. In an open attic, depth is cheap and a lower R-per-inch material is rarely a problem — you simply blow more of it. In a finished 2×6 wall cavity or a shallow floor, depth is fixed by the framing, and only a higher R-per-inch material can reach the target inside that space. Closed-cell spray foam, at roughly double the R-per-inch of fiberglass batts, exists for exactly those tight cavities, and its price per R reflects that advantage.
Choosing a target R-value
The targets below are simplified typical targets, drawn from common North American guidance for homes. They are a sensible place to start, not a substitute for your local building code, which is what a permitted job must meet. For attics, the target rises with climate severity: Zones 1–2 about R-38, Zones 3–4 about R-49, and Zones 5–8 about R-60. Hot climates lose the most heat — and gain the most unwanted heat — through the ceiling, and cold climates simply have more degrees of difference to defend against, for more months of the year.
Walls and floors ask for less, partly because their cavities limit depth and partly because less heat moves through them than through a ceiling. Typical wall targets run from R-13 to R-21 — R-13 fills a 2×4 cavity with a standard batt, and R-21 is the high-density batt made for a 2×6 cavity. Floors over unheated crawl spaces, garages or the outdoors typically target R-25 to R-30. The preset buttons above the calculator set each of these targets in one click, so you can compare depths and bag counts between, say, an R-49 attic done properly and an R-38 attic done cheaply, before you commit to either.
Topping up existing insulation
Most attics do not need to start again. If the insulation already there is dry, clean and roughly even, new insulation can go straight over the top of it, and the R-values add: R-19 below plus R-30 of new blown-in cellulose on top gives you R-49 in total. That is why the calculator asks for an existing R-value and only works out the difference. Estimate what you have by measuring its depth and multiplying by the R-per-inch of the material that is there — about 3.7 per inch for old cellulose, nearer 2.7 for loose fiberglass — and enter that figure.
There are exceptions. Insulation that is wet, mouldy, pest-damaged or crushed flat has lost most of its value and should come out rather than be buried, because new material over a damp layer traps the problem underneath it. Watch, too, for recessed lights, flues and wiring that the old layer was kept clear of: a top-up must respect the same clearances, and fittings not rated for contact with insulation need covers before you blow over them. Finally, do not compress the new layer by walking on it or storing boards on top — compressed insulation is thinner insulation, and thinner means a lower R than you paid for.
Why air sealing comes before adding depth
Insulation slows heat that conducts through a ceiling. It does almost nothing about warm air that leaks through the gaps in that ceiling — around light fittings, pipes, wires, the attic hatch and the tops of interior walls. That moving air carries heat and moisture straight past your new depth, and in many houses air leakage wastes more energy than thin insulation does. Worse, once you have blown another 8 in over the ceiling, every one of those gaps becomes far harder to reach.
So the order of work matters: seal first, then insulate. While the attic is still accessible, run sealant or foam around penetrations, weatherstrip the hatch, and cap or cover anything that should not be buried. The job takes an hour or two and inexpensive materials, it makes the insulation you are about to add perform at its rated value instead of a fraction of it, and it is the one step in the whole project that cannot sensibly be done afterwards. Depth added over an unsealed ceiling is money spent heating the sky, slowly.
A note for UK readers: U-values
The UK measures the same thing the other way up. Instead of R-value, British guidance uses the U-value — how much heat passes through a structure, in watts per square metre per degree — so lower is better, and a U-value is roughly 1 divided by the total R-value once surface resistances are included. You do not need to convert anything to use this page: the depth maths is identical everywhere. For a UK loft, the usual recommendation is about 270 mm of loft insulation in total, which is why so many British lofts are topped up from an older 100 mm layer between the joists with a second layer laid across them. Enter your existing depth converted at roughly R-3.7 per inch for mineral wool, set a target, and the calculator will tell you how much more to add.
Insulation guides
Prefer to work through the method on paper first? These two guides explain the maths behind this calculator in full.
Frequently asked questions
How much insulation do I need for a 1,000 sq ft attic?
It depends on the target and the material. For R-38 with blown-in cellulose from empty, you need 10.3 in of depth, about 856 cu ft of material and 80 bags of 30 lb. With blown-in fiberglass to the same target you need 14.1 in and 36 bags, because fiberglass installs at a much lower density even though it takes more depth. If the attic already has R-19, you only add the difference, which roughly halves the job.
What R-value do I need?
As simplified typical targets, attics need about R-38 in climate Zones 1-2, R-49 in Zones 3-4 and R-60 in Zones 5-8. Walls usually target R-13 to R-21, limited by cavity depth, and floors over unheated spaces R-25 to R-30. Always check your local building code for a permitted job, because code minimums vary by place and by whether the work is new or a retrofit.
How many bags of cellulose do I need?
Work out the volume first: area in sq ft multiplied by depth in feet. Then bags = volume x 2.8 lb per cu ft / 30 lb per bag, rounded up. A 1,000 sq ft attic at R-38 needs about 856 cu ft, so 856 x 2.8 / 30 = 80 bags. Always check the coverage chart on your product, because densities and bag weights vary by brand and the chart is what the manufacturer guarantees.
Can I add new insulation over old insulation?
Usually yes, and it is the normal way to top up an attic. The old and new R-values simply add together, provided the existing layer is dry, clean and not crushed. Wet, mouldy or pest-damaged insulation should be removed first, and clearances around flues, recessed lights and wiring must be kept. Air seal the ceiling before you add the new layer, because you will not easily reach those gaps again afterwards.
Should I choose batts or blown-in insulation?
Batts suit open wall cavities and regular joist spacing, where they can be cut neatly and stay at full thickness; they are also the simpler DIY material in small areas. Blown-in suits attics, especially over existing insulation and around obstructions, because it flows into gaps and forms a continuous blanket with no seams. Per R-value, blown-in cellulose is often the cheaper attic option, while batts win where the cavity will be opened anyway.
Does more insulation always help?
No - returns diminish. Heat flow is proportional to 1 divided by R, so going from R-19 to R-38 cuts the heat loss through that surface by about half again, while going from R-38 to R-57 saves only about a sixth more, and R-57 to R-76 less still. Each added inch costs roughly the same and saves less than the inch before it. That is why targets cluster where they do: enough depth to capture the big savings, without paying for inches that will never repay you.