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Insulation chart calculator
Put your area, coverage per bag and waste in: 400 square feet at 10% waste with 64 square feet per bag is 6.88 bags exact, order 7.
★ CONSTRUCTION · INTERIOR · INSULATION CHART
Insulation Chart: 7 Bags for 400 Sq Ft at 10% Waste
On the defaults — 400 square feet at 10% waste with 64 square feet per bag coverage — you need 440.00 square feet of insulation coverage, 6.88 bags exact, so order 7 bags. This is the INSULATION version — area times waste divided by coverage, with a lookup table for every common area and waste level — the siding-chart page is the SIDING version — the same divide-and-round-up model counted in siding boxes; the stucco-chart page is the STUCCO version — the same model counted in stucco bags; the plaster-chart page is the PLASTER version — the same model counted in plaster bags; and the joint-compound-chart page is the JOINT COMPOUND version — the same model counted in compound gallons.
What does 6.88 bags, order 7, mean?
It means that 400 square feet of wall and ceiling, once 10% waste is added, needs 440.00 square feet of insulation coverage, which is 6.88 bags at 64 square feet a bag — and since insulation is sold by the whole bag, you order 7. The exact figure and the order figure answer different questions: 6.88 is the arithmetic, 7 is the purchase, and just over a tenth of a bag of headroom is thin — which is exactly why the waste allowance matters here, because a bay full of wiring or a lower coverage eats it at once. The part people get wrong is ordering the exact figure rounded down: 6.88 rounded down is 6 bags, 384.00 square feet of coverage against a 440.00-square-foot need, and the last bay runs short. The other part people get wrong is applying the waste percentage to the bags instead of the area: waste is 10% more surface to cover, not 10% more bags on top of a rounded order, and the two give different answers once rounding enters.
Three things to check before you trust the number
Is 64 square feet per bag really your coverage? Sixty-four square feet is the coverage for the product on the bag, and it is the figure the exact-bags line turns on: the 32-square-foot-coverage branch totals 13.75 bags exact against the 6.88 headline, so halving the coverage doubles the order on 400 square feet. If your batts are thicker or your fill is looser than the bag assumes, your real coverage is lower and your real bag count higher — check the product on the bag against the product you are actually installing.
Is 10% waste enough for your bays? Ten percent is the waste allowance, and it is the figure the needed-area line turns on: the 20%-waste branch totals 7.50 bags exact against the 6.88 headline, so doubling waste adds more than half a bag here and nearly two bags on larger jobs. Smooth open bays at 10% are reasonable; bays full of wiring, blocking, and angles want 15% or 20%, because every cut end and every awkward cavity eats insulation that an open-bay figure never sees.
Did you measure every bay? Four hundred square feet is the area, and it is the figure everything else multiplies: the 800-square-foot branch totals 13.75 bags exact against the 6.88 headline, so a missed wall or ceiling doubles the order. Measure bay by bay, subtract large openings only if you genuinely will not insulate them, and add the rim joists and small cavities — small areas that are easy to forget and add up to a bag on a whole house.
Worked example
On the defaults: 400 square feet, 64 square feet per bag, 10% waste.
- Needed area: 400 × 1.10 is 440.00 sq ft.
- Exact bags: 440.00 ÷ 64 is 6.88 bags.
- Order: 6.88 rounded up is 7 bags — area times the waste factor, divided by coverage, rounded up to order, with a 1e-12 epsilon guard so a true exact fit is never rounded up by float error.
And the other way: the area behind 6.88 bags is 400 square feet, the coverage behind 6.88 bags is 64 square feet per bag, and the waste behind 6.88 bags is 10%. With 0% waste the model honestly shows 6.25 bags exact and an order of 7 — still rounded up, because 6.25 bags cannot be bought as six bags and a quarter.
Every branch, checked
- 200 square feet, 64 square feet per bag, 10% waste: 200 square feet, 64 square feet per bag, 10% waste: 3.44 bags exact — order 4 bags.
- 800 square feet, 64 square feet per bag, 10% waste: 800 square feet, 64 square feet per bag, 10% waste: 13.75 bags exact — order 14 bags.
- 400 square feet, 32 square feet per bag, 10% waste: 400 square feet, 32 square feet per bag, 10% waste: 13.75 bags exact — order 14 bags.
- 400 square feet, 64 square feet per bag, 20% waste: 400 square feet, 64 square feet per bag, 20% waste: 7.50 bags exact — order 8 bags.
The reverses, worked backwards
- Area behind 6.88 bags: 400 sq ft — at 400 sq ft the figures are 6.88 bags exact — order 7 bags.; at the lower neighbor the figures are 6.02 bags exact — order 7 bags.; at the upper neighbor the figures are 7.73 bags exact — order 8 bags..
- Coverage behind 6.88 bags: 64 sq ft per bag — at 64 sq ft per bag the figures are 6.88 bags exact — order 7 bags.; at the lower neighbor the figures are 7.86 bags exact — order 8 bags.; at the upper neighbor the figures are 6.11 bags exact — order 7 bags..
- Waste behind 6.88 bags: 10% waste — at 10% waste the figures are 6.88 bags exact — order 7 bags.; at the lower neighbor the figures are 6.56 bags exact — order 7 bags.; at the upper neighbor the figures are 7.19 bags exact — order 8 bags..
What the defaults assume
The defaults are 400 square feet, 64 square feet per bag and 10% waste, so the headline is one medium wall and ceiling area of insulation at a standard product with a standard waste allowance. The model is a multiplication, a division and a round-up: the needed area is area times one plus waste over one hundred, the exact bags are needed area divided by coverage, and the order is exact bags rounded up to the next whole bag, guarded by a 1e-12 epsilon so a true exact fit orders exactly that count rather than one more on float error. The area reverse is checked at 50 square feet either side, because 50 square feet at 10% waste is 55 square feet, or 0.86 bags; the coverage reverse is checked at 8 square feet either side; the waste reverse is checked at 5 points either side, because 5 points on 400 square feet is 20 square feet, or 0.31 bags. The model does not include vapor barriers, fasteners, or different products for different bays — price those separately.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Needed area | area × (1 + waste ÷ 100) | 440.00 sq ft — 400 × 1.10. |
| Exact bags | needed area ÷ coverage per bag | 6.88 bags — 440.00 ÷ 64. |
| Order | exact bags rounded up (1e-12 epsilon guard) | 7 bags — 6.88 rounds up to the next whole bag. |
| Area behind a target exact figure | exact bags × coverage ÷ (1 + waste ÷ 100) | 400 sq ft — 6.88 × 64 ÷ 1.10. |
| Coverage behind a target exact figure | needed area ÷ exact bags | 64 sq ft per bag — 440.00 ÷ 6.88. |
| Waste behind a target exact figure | (exact bags × coverage ÷ area − 1) × 100 | 10% — (6.88 × 64 ÷ 400 − 1) × 100. |
| Headroom in the order | order − exact bags | 0.12 bags — 7 − 6.88, thin headroom that waste alone must protect. |
| 200 square feet, 64 square feet per bag, 10% waste | same model | 3.44 bags exact — order 4 bags. |
| 800 square feet, 64 square feet per bag, 10% waste | same model | 13.75 bags exact — order 14 bags. |
| 400 square feet, 32 square feet per bag, 10% waste | same model | 13.75 bags exact — order 14 bags. |
| 400 square feet, 64 square feet per bag, 20% waste | same model | 7.50 bags exact — order 8 bags. |
| Area behind 6.88 bags | reverse | 400 sq ft: 6.88 bags exact — order 7 bags. |
| Coverage behind 6.88 bags | reverse | 64 sq ft per bag: 6.88 bags exact — order 7 bags. |
| Waste behind 6.88 bags | reverse | 10% waste: 6.88 bags exact — order 7 bags. |
| 0 square feet | same model | 0.00 bags exact — order 0 bags. |
| Negative area | same model | Area — 0 or more. |
| Zero coverage | same model | Coverage — more than 0. |
| 400 square feet, 0% waste | same model | 6.25 bags exact — order 7 bags. |
Lookup table: bags by area and waste
All rows at 64 square feet per bag. Find your area and waste level, read across to the exact figure and the order.
| Area | Waste | Needed area | Exact bags | Order |
|---|---|---|---|---|
| 100 sq ft | 0% | 100.00 sq ft | 1.56 | 2 bags |
| 100 sq ft | 5% | 105.00 sq ft | 1.64 | 2 bags |
| 100 sq ft | 10% | 110.00 sq ft | 1.72 | 2 bags |
| 100 sq ft | 15% | 115.00 sq ft | 1.80 | 2 bags |
| 200 sq ft | 0% | 200.00 sq ft | 3.13 | 4 bags |
| 200 sq ft | 5% | 210.00 sq ft | 3.28 | 4 bags |
| 200 sq ft | 10% | 220.00 sq ft | 3.44 | 4 bags |
| 200 sq ft | 15% | 230.00 sq ft | 3.59 | 4 bags |
| 400 sq ft | 0% | 400.00 sq ft | 6.25 | 7 bags |
| 400 sq ft | 5% | 420.00 sq ft | 6.56 | 7 bags |
| 400 sq ft | 10% | 440.00 sq ft | 6.88 | 7 bags |
| 400 sq ft | 15% | 460.00 sq ft | 7.19 | 8 bags |
| 800 sq ft | 0% | 800.00 sq ft | 12.50 | 13 bags |
| 800 sq ft | 5% | 840.00 sq ft | 13.13 | 14 bags |
| 800 sq ft | 10% | 880.00 sq ft | 13.75 | 14 bags |
| 800 sq ft | 15% | 920.00 sq ft | 14.38 | 15 bags |
Who uses this
- A homeowner insulating one wall and ceiling area — 400 square feet at 10% waste is 6.88 bags exact, order 7 — the lookup table covers the areas around it without touching the calculator.
- An insulation contractor pricing a job — the coverage and waste branches price a different product or rougher bays at once, so a quote can be adjusted before the bags are ordered.
- A builder ordering for a whole house — the 800-square-foot rows scale the same model, and the reverses recover the area, coverage or waste behind any quoted bag count.
- A DIYer checking a store recommendation — the headroom note shows why 7 bags for 400 square feet leaves thin slack, so a recommendation of 6 bags can be checked rather than trusted.
Accuracy & limitations
Figures are the model’s own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp. The main limitation is the inputs, not the arithmetic: whether the area includes every bay and cavity, whether the coverage figure matches the product actually installed, and whether the waste allowance suits the bay condition are all judgement calls, and this page prices the figures you enter without checking them. This page is a calculation tool, not building advice.
Cite this calculator
400 square feet at 10% waste with 64 square feet per bag needs 440.00 square feet, 6.88 bags exact: order 7 bags. Source: Bright Side Kit, https://brightoolbox.com/insulation-chart/, accessed 29 October 2026.
Frequently asked questions
Why order 7 bags when the exact figure is 6.88?
Because insulation is sold by the whole bag and you cannot buy eighty-eight hundredths of a bag. Rounding down to 6 would leave 56.00 square feet uncovered; rounding up to 7 covers the full need with just over a tenth of a bag of headroom, which is why the waste allowance must be honest on this material.
When does the order need no rounding up?
When the needed area divides evenly by the coverage: 200 square feet at 0% waste is not an exact fit at 64 square feet a bag, but 320 square feet at 0% waste is exactly 5.00 bags, so the order is 5. Exact fits are the exception, not the rule, and they vanish the moment waste is added.
Should waste apply to the area or to the bags?
To the area. Waste is extra surface to cover — cutting, fitting, uneven cavities — so it belongs in the needed area before dividing by coverage. Applying waste to the bags after rounding gives a different, usually smaller, answer that understates the need.
What coverage figure should I use?
The one on the bag or product sheet for the product you are actually installing. A bag rated at 64 square feet for one product may cover far less with thicker batts or a looser fill, and using the wrong product is the fastest way to run short mid-bay.
Do I subtract windows and doors?
Only openings you genuinely will not insulate. A door or window opening saves the flat area but adds framing to work around, so subtracting the full opening without adding the fitting back understates the job. When in doubt, leave small openings in and treat the saving as extra headroom.
How is an insulation chart different from a siding chart?
Only the material changes: insulation at 64 square feet per bag against the siding page at 200 square feet per box. The model — area times waste divided by coverage, rounded up — is identical, which is what makes the chart pages comparable.