Example: 300 ft of six-inch flat trim
At a hypothetical 350 ft²-per-gallon coverage, six-inch trim has 700 ft-per-gallon linear coverage. For 300 ft, two coats and 10% allowance, demand is 300 × 2 × 1.10 = 660 linear feet of coverage. 660 ÷ 700 = 0.9429 US gallon; the calculator reports one gallon. The flat painted area before coats is 150 ft².
List the actual runs receiving this finish
Measure baseboards, door casing, window casing and other trim separately before combining them. Record which faces will be coated. A run measured along the wall does not describe returns, exposed ends or both sides of a free-standing piece.
Keep different products and colours on separate lines even when they are all called trim paint. This allows one room’s finish to change without silently changing another. Material length estimates for new moulding also include cutting waste that may not represent painted surface; use the installed paint scope here.
Convert area coverage to linear coverage
For a simple flat strip, painted area equals run length multiplied by painted width in the same length unit. Therefore linear coverage equals square-foot coverage per gallon divided by painted width in feet. Wider trim uses more paint per foot than narrow trim at the same product coverage.
A six-inch painted width is 0.5 ft. If an illustrative product rate is 350 ft² per gallon, the equivalent linear rate is 350 ÷ 0.5 = 700 ft per gallon for one coat. Do not divide by six without first converting inches to feet.
Treat shaped mouldings carefully
Decorative profiles expose more surface than a flat face of the same projected width. Use a documented effective painted width or a measured trial rate when the difference matters. Do not present a flat-strip conversion as an exact profile-surface calculation.
The BEHR estimator identifies trim separately from walls. The practical lesson for your takeoff is to maintain that distinction. A wall roller coverage assumption and a brush-applied trim rate may not be interchangeable, even when the same volume unit appears on both cans.
Combine only compatible coverage bases
If baseboards and casing have different effective widths, calculate each group with its own linear coverage. Add their unrounded paint needs before purchasing compatible cans. Averaging the widths without weighting by run length can bias the total.
Enter the planned coats and an explained loss allowance once. Primer, preparation and final enamel remain separate scopes. The calculator rounds to whole US gallons; a local supplier may offer smaller or litre-sized containers, so map the calculated volume to available packs instead of assuming the tool’s container is universal.
Do it in order
- Measure installed runs by profile and finish.
- Convert painted width into feet or metres.
- Derive a matching linear coverage rate from documented area coverage.
- Apply coats and loss, then combine compatible paint volumes.
Planning boundary
The flat-strip conversion estimates consumption. It does not select a coating or determine preparation requirements.
Sources checked
Can I enter 350 when the can says 350 ft² per gallon?
Not into a linear-coverage field. Convert it using the painted width first, as shown in the example.
Can different trim widths share one line?
Only if the entered rate correctly represents the combined scope. Separate groups are easier to verify.