Perimeter method
Do not measure the room; measure the walls. They are not the same thing in any room with an alcove, a chimney breast, a bay or a bulkhead.
Walk the room in one direction with a tape and write down every wall length in order. A simple 12 by 14 bedroom gives you 14, 12, 14, 12 — a perimeter of 52 linear feet. A room with a 3 foot deep, 6 foot wide chimney breast gives you eight numbers instead of four, and a perimeter of 58 feet. That extra 6 feet is half a stick of trim and two more corners.
Write the numbers in the order you walked them, not sorted. When you come to cut, you want to work round the room in sequence so each piece is fitted against the one already up, and a sorted list loses that information.
- Start at the door and walk one way round the room.
- Measure each wall at floor level, tape flat against the wall, not across a corner.
- Note every opening as you pass it, with its width.
- Mark which corners are inside and which are outside.
- Add the wall lengths for the gross perimeter.
- Subtract the openings to get the net length of trim.
Once you have the perimeter, the baseboard calculator handles the rest: openings, waste and the rounding to stock lengths, plus a plan showing where each stick lands.
Subtracting openings
Deduct the width of anything with no baseboard running across the floor. That is doorways and cased openings.
| Opening | Width | Subtract? |
|---|---|---|
| Interior door | 2 ft 6 in to 3 ft | Yes |
| Double or french doors | 5 ft to 6 ft | Yes |
| Cased opening | 3 ft to 6 ft | Yes |
| Bifold closet | 4 ft to 6 ft | Yes, across the opening only |
| Fireplace or hearth | Varies | Yes, if it meets the floor |
| Radiator or fridge recess | Varies | No — trim runs behind |
| Built-in wardrobe | Varies | Yes, if it is fixed to the floor |
Two doors in a 52 foot perimeter removes 6 feet, so you are trimming 46 linear feet. That is the number the waste factor applies to.
Mitre waste allowance
Ten percent for a normal room. Every mitre consumes length at the end of a stick — a 45 degree cut on a 5 1/4 inch tall baseboard uses about 5 inches of the board's length — and a blown cut costs the whole piece.
Go to 15% if the room has more than two outside corners, because outside mitres burn length at both ends of both pieces and they are the cuts most likely to need a second attempt. Bay windows, columns and boxed-in ducts all fall into this category.
So 46 linear feet becomes 51 with waste. That is the number you divide by the stock length. Round that result up, then add one more stick if the room has any outside corner at all: the first outside mitre you cut will probably be backwards, and everybody's is.
Stock length rounding
Trim comes in 8, 12 and 16 foot sticks, and in 10 and 14 for solid softwoods. Longer is better, and the reason is joints: a 14 foot wall run from a 16 foot stick is one clean piece, while the same wall from 8 foot stock needs a scarf joint in the middle. A scarf joint is two 45 degree cuts overlapped, and no matter how carefully it is glued and sanded, it will telegraph through the paint in raking light within a year as the two pieces move differently.
| Net trim needed | With 10% waste | 8 ft sticks | 12 ft sticks | 16 ft sticks |
|---|---|---|---|---|
| 30 ft | 33 ft | 5 | 3 | 3 |
| 46 ft | 51 ft | 7 | 5 | 4 |
| 60 ft | 66 ft | 9 | 6 | 5 |
| 80 ft | 88 ft | 11 | 8 | 6 |
| 120 ft | 132 ft | 17 | 11 | 9 |
Assign the longest sticks to the longest walls first, then use the offcuts on the short walls. Doing that on paper before you cut is worth more than any amount of care at the saw.
Corners and returns
Inside corners get coped, not mitred. Cut the first piece square into the corner. Cut the second at 45 degrees to expose the profile line, then follow that line with a coping saw, back-cutting slightly. The result interlocks and stays tight when the framing moves seasonally. A mitred inside corner opens within one heating season and no amount of caulk keeps it closed.
Outside corners get mitred, and the angle is rarely 45. Check the corner with an angle finder: a wall at 91 degrees needs two 45.5 degree cuts. Cut both pieces long, dry-fit them, then trim to the line.
Open ends get a return. Where a run stops in mid-air — beside a stair stringer, at a cased opening with no casing to die into — mitre the end at 45 degrees, then cut a small triangle from an offcut to turn the profile back to the wall. Two cuts, thirty seconds, and it is the single detail that most separates finished carpentry from unfinished.
Nail into studs, not just drywall: two nails per stud, one low into the plate and one high into the stud, angled slightly in opposite directions so the piece cannot pull off the wall. Baseboard nailed only to drywall lifts the first time a vacuum hits it.
One last piece of planning: prime and put the first coat of paint on the sticks before they go up, then caulk the top edge and the corners and apply one finish coat in place. It is faster, it is cleaner, and the coats work out as described in how many coats of paint. If the room is being boarded first, the sheet and compound quantities are in how many sheets of drywall.
Frequently asked questions
How do I measure baseboard for a whole house?
Room by room, never as one total. Measure each room perimeter, subtract its openings, and keep the lists separate so you can order by stock length and cut the long walls from the long sticks.
Do I add waste to trim?
Ten percent for a normal room, 15% when there are several outside corners. Mitres eat length at both ends of a cut and a blown mitre costs you the whole piece.
What is a return and when do I need one?
A small mitred piece that turns the profile back into the wall where a run ends in open air, such as beside a stair stringer or a cased opening. It is two cuts on a short offcut and it is the detail that separates finished from unfinished.
Should I paint the trim before or after I install it?
Before, for the primer and the first coat, with the sticks laid flat on sawhorses. Install, fill the nail holes, caulk the top edge, then one finish coat in place. It is faster than cutting in twice and the finish is better because the first coat had no gravity working on it.