THE MATH, MADE VISIBLE
How it works
Perimeter = 2 × (room length + room width), or enter the perimeter directly for rooms that are not rectangles. Net run = perimeter − (door openings × width per opening). Order length = net run × (1 + allowance ÷ 100). Boards = round up(order length ÷ board length). Cost = boards × your price per board. Nail planning example: nail spots = round up(net run ÷ spot spacing), nails = spots × nails per spot.
Real project examples
A 12 × 10 ft bedroom
The perimeter is 2 × (12 + 10) = 44 ft. One 36 in door opening leaves 41 ft of baseboard on the wall. Add 10%: 45.1 ft. In 12 ft boards, that is 45.1 ÷ 12 = 3.8, so buy 4 boards (48 ft). The nail planning example, 2 nails every 16 in, gives 31 spots and 62 nails.
A 14 × 12 ft room with two doors
The perimeter is 52 ft. Two 36 in openings take off 6 ft, leaving 46 ft. Add 10%: 50.6 ft. In 16 ft boards, 50.6 ÷ 16 = 3.2, so buy 4 boards (64 ft). At $20 per board, the boards cost $80. The same order in 8 ft boards would be 7 boards.
Before you buy
- This Old House advises measuring the room and adding 10% for waste. That is the default allowance here; raise it for rooms with many outside corners, short walls or stain-grade trim where a bad cut cannot be hidden.
- Measure the walls at floor level, where the baseboard goes, and include closets, alcoves and bump-outs that will be trimmed. For an irregular room, add up the wall lengths and enter that as the perimeter.
- Subtract only openings where the baseboard stops, such as doorways and cased openings. The 36 in default is a planning example; measure the actual opening between the casings.
- Longer boards mean fewer joints. Where a wall is longer than one board, plan scarf joints, as Lowe’s and This Old House describe, and put them where they are least visible.
- The nail count is a planning example, not a fastening schedule. Lowe’s says to drive finishing nails into the wall studs every 16–18 in; This Old House nails near the top and bottom edges. Follow the trim maker’s instructions for your profile.
- Buy all the trim from the same run where you can, and let wood trim acclimate in the room for a few days before you cut it.
Questions worth asking
How do I calculate how much baseboard I need?
Add up the length of every wall where the trim goes (for a rectangle, 2 × (length + width)), subtract door openings, then add a cutting allowance. Divide by the length of board you will buy and round up.
Should I subtract doors and windows?
Subtract doorways and any opening where the baseboard stops. Do not subtract windows unless they reach the floor, because the baseboard runs underneath them.
Which board length should I buy?
Whatever your store stocks and you can transport. Longer boards cover long walls with fewer joints; shorter boards are easier to handle and can waste less in a room of short walls. Compare the board count for 8, 12 and 16 ft with the selector.
How many nails do I need?
The calculator shows a planning example: 2 nails at a spot every 16 in along the net run. Change the spacing and nails per spot to match your studs and your trim. Buy a box; the count just tells you which box size.
Sources & assumptions
This Old House · how to install baseboards (10% extra)Lowe’s · baseboard moulding installation guide (nailing)NIST · unit conversion factorsLast reviewed October 1, 2026. Planning defaults are editable examples. Product labels and supplier information override them.