Floor Plan Square Footage Calculator
Calculate floor plan square footage by measuring each room separately and summing the totals. Use the multi-segment tool to add all rooms in one calculation.
Floor Plan Calculator
Room-by-room measuring
For an existing home: measure each room's interior dimensions and label them on a floor plan sketch. Sum all rooms, then add hallway and stair landing areas.
A typical 3-bedroom 2-bath ranch has: living room (~250 sq ft), kitchen (~200), dining (~150), 3 bedrooms (~150 each), 2 bathrooms (~50 each), hallways (~80) = roughly 1,330 sq ft of interior + walls = 1,500-1,600 sq ft gross.
Interior vs. gross floor area
Adding up interior room dimensions gives you "carpetable" or net floor area. The official ANSI Z765 square footage measures to the exterior wall surface and includes the walls themselves.
The difference between interior-only and gross-with-walls is typically 10-12% for residential construction. A house with 1,500 sq ft of net interior space usually has 1,650-1,700 sq ft of ANSI gross.
Reading dimensions from a floor plan
Floor plans use different dimension conventions and you need to know which one to read.
Inside dimensions (room only, finished wall to finished wall): the smaller number. Used for room labels like '12-0 × 14-0' meaning 12 ft 0 in by 14 ft 0 in of usable floor.
Center-line dimensions (centerline of one wall to centerline of another): used in architectural drawings for overall house perimeter.
Outside dimensions (exterior face to exterior face): the largest number. Used for site planning.
For ANSI Z765 residential square footage: use outside dimensions, then subtract typical wall thicknesses (6 inches per exterior wall, 4 inches per interior wall) to get usable interior space.
Room-by-room calculation from a plan
Most accurate approach: list every room on the floor plan with its dimensions, calculate each room's area, sum the rooms, add hallways and circulation, compare to the floor plan's labeled total.
Worked example, 3-bedroom 2-bath: master bedroom 14 × 16 = 224. Bed 2: 12 × 12 = 144. Bed 3: 11 × 12 = 132. Master bath: 8 × 10 = 80. Bath 2: 5 × 8 = 40. Kitchen: 12 × 14 = 168. Living room: 15 × 20 = 300. Dining: 12 × 12 = 144. Hallway/foyer: ~80. Closets: ~60. Total: 1,372 sq ft.
Compare to the floor plan's listed total. They should agree within 5%. If not, you've missed a room or there's a measurement issue with the plan.
Floor plans vs as-built reality
Designed dimensions on drawings are typically accurate to ±1 inch. As-built construction varies 1-3% from drawings due to framing tolerances.
When buying from a floor plan (new construction): the actual house may be 50-100 sq ft different from the plan. This is normal. Verify after framing if it affects your purchase decision.
Trim and crown molding aren't shown on most floor plans but slightly reduce usable space. Door swings are hatched on plans but the swing area is still usable floor (a closet floor doesn't shrink because a door opens into it).
Stairs are shown as triangles with arrows indicating up/down. The floor space directly under the stair counts toward the lower floor.
Pro tips
Use blueprints if available
Architectural drawings list room dimensions and total square footage. Cross-check the blueprint number against your physical measurement.
Don't double count walls
When adding room areas to get a total, you're measuring interior area only. Don't also add the wall thickness - that comes out automatically when measuring exterior to exterior.
Stairs count once
A staircase counts as floor area on the lower floor only - the upper-floor "open to below" doesn't count.
Closets are part of the room
For interior measurement, closets are usually included with the bedroom. For ANSI gross, they're part of the building footprint.