Insulation Calculator (R-Value, Depth and Bags)
Choose a target R-value and the insulation type, enter any existing insulation, and use the coverage chart on your bag for the bag count.
How it works
Existing R ≈ existing depth × R per inch. R to add = target R − existing R. Inches to add = R to add ÷ R per inch (the higher R per inch gives the smaller depth). Bags = area ÷ sq ft per bag at the target R from the manufacturer's coverage chart, rounded up.
Worked example: DOE: R-38 ÷ 3.8 per inch = 10 in of cellulose, ÷ 3.2 = 11.9 in. With 4 in already in place (≈ R-12 by ENERGY STAR's × 3 estimate) and a target of R-49, add R-37: about 9.7 to 11.6 in of cellulose.
Assumptions
- Target R-values are ENERGY STAR's recommended attic levels for retrofitting existing wood-framed homes (for example R-49 in Zones 2-3 and R-60 in Zones 4-8 for an uninsulated attic). ENERGY STAR says its table is based on the 2021 IECC. They are shown as general information; your local code and climate zone decide what applies, so check local codes.
- R per inch ranges are DOE figures for settled loose-fill (via the Building America Solution Center): cellulose 3.2-3.8, fiberglass 2.2-2.7, rock wool 3.0-3.3. Products differ; the bag label's chart is the figure to use.
- Existing insulation default R-3 per inch follows ENERGY STAR's quick estimate (depth in inches × 3). Compressed, wet or damaged insulation performs worse.
- Bag coverage depends on the product and target R; this page does not guess it. Air sealing, baffles and clearances around recessed lights and chimneys are not covered; follow the manufacturer's instructions.
Frequently asked questions
How many inches of insulation for R-38?
DOE's loose-fill table lists about 10-12 in of cellulose, 14-17 in of fiberglass or 11.5-13 in of rock wool. That is R-38 divided by each material's R per inch.
What R-value should my attic have?
ENERGY STAR's table suggests R-30 to R-60 for uninsulated attics, depending on climate zone (R-60 in Zones 4-8). Treat it as general information and check local codes.
How do I estimate the R-value of my existing insulation?
ENERGY STAR suggests multiplying the depth in inches by about 3, since common insulation types run about R-3 to R-3.5 per inch.
Related calculators
- Drywall calculator (sheets)
- Square footage calculator
- Roofing calculator (squares and bundles)
- Siding calculator (squares)
Sources
Formulas and reference values are from these sources. Page content last updated 2026-10-19.
- ENERGY STAR: Recommended home insulation R-values (attic levels by climate zone)
- ENERGY STAR: Well-insulated and sealed attic (common insulation is about R-3 to 3.5 per inch)
- U.S. DOE Building America Solution Center (PNNL): Loose-fill insulation (R per inch, inches for R-38)
- NIST Guide to the SI (SP 811), Appendix B.9: conversion factors (foot = 0.3048 m, ton, pound)