The formula
Area = length × width To buy = area × (1 + extra %) Boxes = to buy ÷ coverage per box, rounded up Underlay = area × 1.05 ÷ roll coverage, rounded up
Example: a 12 × 15 ft bedroom is 180 square feet. With 10% extra that is 198 square feet, so at 20 square feet per box you need 10 boxes (200 square feet), plus 2 rolls of underlayment and about 54 ft of baseboard or quarter round.
Quick answers for common rooms
| Room | Area | Boxes (20 sq ft) | Flooring bought | Underlayment rolls |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 6 | 120 sq ft | 2 |
| 10 × 12 ft | 120 sq ft | 7 | 140 sq ft | 2 |
| 12 × 12 ft | 144 sq ft | 8 | 160 sq ft | 2 |
| 12 × 15 ft | 180 sq ft | 10 | 200 sq ft | 2 |
| 15 × 20 ft | 300 sq ft | 17 | 340 sq ft | 4 |
| 20 × 25 ft | 500 sq ft | 28 | 560 sq ft | 6 |
How much extra to buy
- 5 to 10% for planks laid straight in a simple room.
- About 15% for diagonal layouts or rooms with many corners and doorways.
- 15 to 20% for herringbone and chevron patterns.
Buy all the boxes at once from the same production lot, and keep a spare box for future repairs. The coverage per box is printed on the label; it varies by product, so always use the figure on yours.
Before you install
- Leave the boxes in the room to acclimate for as long as the maker says, often 48 hours.
- Floating floors need an expansion gap at the walls, usually 1/4 to 3/8 inch, hidden by baseboard or quarter round.
- Check whether your flooring has a pad attached. If it does, extra underlayment is usually not allowed; a vapor barrier may still be needed over concrete.
Sources and assumptions
Coverage per box and underlayment rules come from the flooring maker’s label and installation guide. The 5% extra on underlayment covers seams and trimming at the walls.
Questions
How many boxes of flooring do I need for a 12 × 15 room?
With 10% extra and 20 square feet per box, 10 boxes. Check the coverage on your box, which varies from about 16 to 30 square feet.
How much extra flooring should I buy?
About 10% for straight layouts, 15% for diagonal layouts or rooms with many corners, and 15 to 20% for herringbone.
Do I need underlayment under laminate or vinyl plank?
Most laminate needs a foam underlayment unless one is attached. Many vinyl planks have a pad attached and need no extra underlayment; follow the maker’s instructions.
References and review
Units convert with exact definitions: 1 ft = 12 in = 0.3048 m, 1 yd³ = 27 ft³. Results round up to whole bags, sheets and pieces so you don’t come up short. Last reviewed October 20, 2026. If your bag, pallet or supplier lists a different figure, use theirs: change the matching field above.