Blown Cellulose Insulation Calculator

Calculate added R-value and whole bags from attic area and entered product coverage at that added R. A target already met needs zero additional bags; no generic depth is inferred.

Interactive calculator · Method INSULATION-2.2 · Last reviewed: 2026-11-04

Estimate project quantities and cost ranges

This home-project calculator uses dimensions, waste factors, multipliers, or unit costs to create an early planning estimate. Treat the output as a pre-bid planning number, not a contractor quote.

Your inputs

Try an example

Area (sq ft)

Target R-value

Existing R-value

Bag coverage at added R (sq ft/bag)

Cost per bag (USD)

Default worked example. Edit the inputs to calculate your scenario.

Default worked example

Bags needed 25

Added R-value 28

Entered manufacturer coverage 40 sq ft/bag at added R

Entered material cost $450.00

Area 1,000 sq ft

Target R 38 − existing R 10 = added R 28. Use manufacturer bag coverage for that added R-value: ceiling(1,000 ÷ 40) = 25 bags.

How to use the calculator

Measure area and existing R-value. Choose a target and enter actual manufacturer coverage per bag for the added R-value, not total existing plus added insulation.

What the result means

A target already met requires no additional bags in this model. Product coverage charts determine bag count when additional R is needed. Rounded area or R-value cards can look equal at a boundary even when the unrounded entries differ. Print or copy the full inputs with the result; the bag count uses those exact entries.

Formula and units

Added R = maximum(0, exact target R − exact existing R). When added R is zero, no bags are needed. Otherwise whole bags = ceiling(exact entered area / entered manufacturer coverage per bag at that added R). Cost = whole bags × entered USD price. Comparisons and ceilings use the full entered decimals before display rounding.

Worked examples

Area (sq ft): 1000 sq ft; Target R-value: 38 R-value: Area (sq ft): 1000 sq ft; Target R-value: 38 R-value; Existing R-value: 10 R-value; Bag coverage at added R (sq ft/bag): 40 sq ft per bag; Cost per bag (USD): 18 USD per bag.
Example result: Bags needed: 25; Added R-value: 28; Entered manufacturer coverage: 40 sq ft/bag at added R; Entered material cost: $450.00; Area: 1,000 sq ft.

Area (sq ft): 1500 sq ft; Target R-value: 57 R-value: Area (sq ft): 1500 sq ft; Target R-value: 57 R-value; Existing R-value: 15 R-value; Bag coverage at added R (sq ft/bag): 40 sq ft per bag; Cost per bag (USD): 18 USD per bag.
Example result: Bags needed: 38; Added R-value: 42; Entered manufacturer coverage: 40 sq ft/bag at added R; Entered material cost: $684.00; Area: 1,500 sq ft.

Limitations and common mistakes

Settled thickness, product density, air sealing, moisture, fire clearances and installation rules require product instructions and site assessment. No universal R-value per inch is calculated. Keep target R, existing R and the coverage chart on the same R scale. This tool performs no RSI/R conversion. Modeled material cost before rounding must not exceed USD 1 trillion and whole counts must remain safe integers. Tiny positive quantities and subcent USD values use three significant scientific digits; other displays round half-up. Inputs retain up to 64 decimal characters and exponents from -308 to 308.

Do not use coverage for total target R when the existing layer already contributes R. Do not infer settled depth from a generic constant.

Sources and assumptions

Area and bag arithmetic are mathematical. Coverage and settled depth must come from the product chart; no universal R per inch is assumed.

Method INSULATION-2.2; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.