Understanding your result
The headline is the gap to leave at every wall, with the run limit and the number of transition strips beneath it. The table repeats the gap for pipes, cabinets and door frames, gives the room’s dimensions against the run limit, counts the transitions in each direction, and names the trim that will cover the gap.
The gap at each wall comes from the material. Floating floors (laminate, engineered, vinyl plank) use the common manufacturer figures; nailed solid wood uses the NWFA rule of a gap equal to the board thickness. If you chose the humidity-swing option, floating-floor gaps are increased by half, because a floor that moves more needs more room. The run limit applies to floating floors only: past it, the floor must be broken by a T-moulding into separate sections, and the table counts how many breaks the room needs along each dimension.
For solid wood, the table adds the possible seasonal width change of the floor if every board moved freely. It uses the dimensional change coefficient for flatsawn red oak from the USDA Wood Handbook. The figure is a worst case, since nailed boards share the movement between themselves, but it explains why the gap is a rule and not a courtesy. The row count that the gap feeds into is on the plank layout calculator; the T-mouldings and reducers are counted on the thresholds and transitions calculator.
How we calculate this
The floating-floor figures are the ones most manufacturers print: 1/4 in for vinyl plank, 3/8 in for laminate, 1/2 in for floating engineered wood, with run limits of about 50, 40 and 40 ft. Your carton may differ, which is what the custom option is for. The coefficient 0.00369 is the change in width per 1% change in moisture content for flatsawn red oak (USDA Forest Products Laboratory, Wood Handbook, Table 13-5); a moisture swing of 4 points is typical of a conditioned house through the year, 8 of a house that is closed or poorly controlled. The guide to expansion gaps explains how wood and core materials move, and the standard sizes reference lists trim sizes that cover each gap.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Solid wood gap | Flooring thickness (3/4 in) | NWFA installation guidelines; Hardwood Floors magazine “Tech Talk” on perimeter expansion space (2025) |
| Laminate gap and run | 3/8 in, 40 ft | Typical laminate installation instructions (Pergo, Mohawk); 30 to 40 ft run limits are common |
| Engineered (floating) gap and run | 1/2 in, 40 ft | Typical floating engineered wood instructions |
| Vinyl plank gap and run | 1/4 in, 50 ft | Typical rigid-core vinyl plank instructions |
| Humidity swing factor | 1.5 × the gap | A judgement for houses outside 35 to 55% RH; the warranty usually requires that range |
| Red oak movement | 0.00369 per 1% MC, flatsawn | USDA Forest Products Laboratory, Wood Handbook (2021), Table 13-5 |
Assumptions last reviewed October 8, 2026.
The calculator does not account for radiant heating under the floor, which most makers treat with their own limits and acclimation rules, or for very large commercial rooms, where makers specify gaps by the run length. It uses red oak for the movement figure; maple, hickory and many exotics move more, quartersawn wood less. And it assumes the trim is attached correctly, to the wall and never through the floor, which is the half of the rule people break. The guide comparing solid, engineered and laminate covers how each material behaves in a humid or dry climate.
Two worked examples
A 58 × 24 ft open plan in vinyl plank
Floating rigid-core vinyl plank, conditioned house.
- Gap: 1/4 in at every wall and fixed object
- Run limit: 50 ft; the 58 ft length needs ceil(58 ÷ 50) − 1 = 1 T-moulding; the 24 ft width needs none
- Trim: baseboard alone covers 1/4 in
One break across the length, best placed at a doorway or under the edge of a kitchen island if one falls near the middle. Each half of the floor then floats separately with its own 1/4 in gap at the moulding.
A 22 × 16 ft room of solid oak in a lake house
3/4 in solid wood, nailed, house closed through the winter.
- Gap: 3/4 in at every wall, no run limit
- Possible width change across 16 ft if every board moved freely: 192 in × 0.00369 × 8 = 5.67 in
- Trim: quarter round or baseboard plus shoe moulding to cover 3/4 in
Nearly six inches of possible movement across a 16 ft room is why a closed lake house is the classic setting for cupped and gapped floors. The nails share most of it between the boards; the gap takes the rest, and a dehumidifier over the winter takes away the cause.
Where to find your inputs
Flooring. The product type on the carton. If your carton gives its own gap and run limit, choose the custom option and type them.
Room length and width. Length along the planks, width across them. The run limit applies to both directions for floating floors.
Humidity. A house heated and cooled all year is “conditioned”. A cottage closed for months, a basement without a dehumidifier or a sunroom is a “big swing”.
Thickness. For solid wood, on the carton; 3/4 in is standard, 5/16 and 1/2 in solid also exist.
Common mistakes
- No gap at the door frames. Undercut the casing so the floor slides under it with the same gap.
- Trim nailed through the floor. Nail baseboard and quarter round to the wall. A pinned floating floor buckles.
- Cabinets on a floating floor. Fixed cabinets and islands go on the subfloor; the floor stops short of them.
- One gap for every material. Solid wood, laminate and vinyl each have their own figure. Check the carton.
- Ignoring the run limit. A floating floor longer than the limit buckles in the middle in summer.
- Spacers left in. Pull them before the trim goes on, or the gap is filled.
Questions people ask
- How big should the expansion gap be for laminate?
- Most laminate makers specify 1/4 to 3/8 inch at every wall and fixed object, and some ask for more in large rooms. This calculator uses 3/8 inch for a room held at 35 to 55% relative humidity and half as much again where humidity swings. The carton's instructions override any general figure.
- What expansion gap does solid hardwood need?
- The NWFA rule is a gap equal to the thickness of the flooring, so 3/4 inch for standard 3/4 inch solid wood, at every wall and vertical obstruction. It matters most on the walls parallel to the boards, because wood moves across its width and hardly at all along its length.
- Do vinyl plank floors need an expansion gap?
- Yes. Rigid-core and click vinyl plank expands and contracts with temperature more than with humidity, and most makers specify 1/4 inch at every wall. Sunlit rooms and rooms over radiant heat move the most. Glue-down vinyl is held by the adhesive and needs less.
- When does a floating floor need a T-moulding?
- When a run exceeds the maker's limit, commonly 30 to 40 feet for laminate and engineered wood and around 50 feet for vinyl plank, and at most doorways between rooms. Each section then floats separately with its own gap. A 58 foot run of vinyl plank needs one break.
- How do I hide the expansion gap?
- A baseboard covers up to about 1/2 inch; quarter round or shoe moulding added to the baseboard covers 3/4 inch. Trim is nailed to the wall or the baseboard, never through the floor, or the floor cannot move. Door casings are undercut so the floor slides under them.
- Does the gap apply under kitchen cabinets?
- Fixed cabinets and islands are not set on a floating floor; the floor stops short of them with the same gap, covered by a toe-kick trim. Setting a heavy island on a floating floor pins it, and the floor buckles elsewhere.