The Framing of a Room
When painting a room, the broad drywall surfaces are only half the job. The architectural "Trim"—which includes the baseboards running along the floor, the casing wrapped around the doors and windows, and the crown molding touching the ceiling—requires an entirely different approach, different paint, and a different set of mathematical calculations.
While you buy wall paint based on total square footage, trim is almost always calculated by Linear Footage (the straight-line distance around the room).
However, trim varies wildly in size. A simple modern baseboard might only be 3 inches wide, while a massive Victorian crown molding combined with tall baseboards and wainscoting could equal over 15 inches of total trim width per linear foot. Accurately estimating trim paint requires converting that linear perimeter into a true square footage area.
Semi-Gloss Enamel vs. Flat Wall Paint
You should almost never paint your trim with the same paint you used on your walls.
Walls are typically painted with a "Flat" or "Eggshell" finish to hide drywall imperfections. Trim, however, takes massive physical abuse. Baseboards are constantly kicked by shoes, hit by vacuum cleaners, and scuffed by moving furniture. Door casings are constantly grabbed by dirty hands.
Trim MUST be painted with a highly durable Semi-Gloss or High-Gloss Enamel. Enamel dries to a hard, plastic-like shell that is highly resistant to impact. The glossy finish allows you to scrub the trim with harsh chemical cleaners without washing the paint off the wood.
Because high-gloss enamel is significantly more expensive than flat wall paint (often $1 to $1 per gallon), accurately estimating the required amount is critical to your budget.
How to Calculate Trim Paint
To calculate the required gallons of trim paint, you must measure the total linear length of all the trim, factor in its average width, convert it to square feet, and divide by the standard coverage rate of a gallon of paint (roughly 350 sq ft).
The Formula
- Measure the total Linear Perimeter of the baseboards in the room.
- Measure the total Linear Perimeter of the crown molding (if applicable).
- Measure the linear length of the casing around all doors and windows (A standard door has about 17 linear feet of casing).
- Add all these numbers together to find the Total Linear Feet.
- Determine the Average Width of all this trim in inches.
- Multiply the Total Linear Feet by the Average Width to find the Gross Area in "inch-feet."
- Divide by 12 to convert the area into true Square Feet.
- Multiply by the Number of Coats (Trim almost always requires 2 coats for a perfectly smooth finish).
- Divide by 350 (the standard coverage of 1 gallon of premium enamel).
Total Gallons = (((Linear Feet × Average Width) ÷ 12) × Coats) ÷ 350
Example Calculation
You are painting the trim in a massive living room. You have 100 feet of baseboards, 100 feet of crown molding, and 50 feet of door/window casing. The total linear length is 250 feet. The average width of all the trim is 5 inches. You are doing 2 coats.
- Multiply Length by Width:
250 × 5 = 1,250 inch-feet - Convert to Square Feet:
1,250 ÷ 12 = 104 square feet of actual wood surface - Multiply by 2 Coats:
104 × 2 = 208 square feet to be painted - Divide by Coverage:
208 ÷ 350 = 0.59 Gallons
You should buy 1 Gallon of high-gloss trim paint for this room.