Five AI models gave 345 replies to ten room-area problems; the worst was 0.26% off. The real risks: fractions, one blank reply, and inside vs outside walls.
We gave five current AI models ten room-area problems, from a plain 12' 7" by 15' 3" bedroom to an L-shaped room described only by its wall lengths, and the furthest any answer in 345 replies landed from the exact area was 0.26%. The arithmetic held up. The mistakes worth knowing about were in format, in one answer that never arrived, and in which square footage you are actually asking for.
That is a better result than we expected. Feet and inches mixed with meters is easy to get wrong, and a wrong square footage turns into a wrong flooring order or a wrong price per foot. So we tested it properly, with exact answers worked out in code and every raw reply kept.
How we tested
On 10 October 2026 we called five models through an OpenAI-compatible API gateway, using the ids it lists: claude-opus-5-5 and claude-haiku-5-5 (Anthropic), gemini-3.1-pro-low and gemini-3.8-flash-high (Google) and deepseek-flash (DeepSeek). This is API access, not the Claude, Gemini or DeepSeek chat apps. We sent one user message with no system prompt of our own, left temperature at its default and capped each reply at 16,000 tokens.
The ten problems, with answers computed exactly in a script:
| Problem | Exact answer |
|---|---|
| Bedroom, 12 ft 7 in × 15 ft 3 in | 191.90 sq ft |
| L-shaped room, outside walls 21 ft and 16 ft 6 in, arms 9 ft 3 in and 11 ft | 274 sq ft |
| 13 ft 4 in × 14 ft room plus a trapezoid bay (8 ft, 4 ft 6 in, 2 ft 6 in deep) | 202.29 sq ft |
| 14 ft 2 in × 12 ft 10 in bedroom minus a 2 ft 4 in × 6 ft 8 in closet | 166.25 sq ft |
| Mixed units: 4.2 m × 11 ft 9 in | 161.91 sq ft |
| 12 × 10 ft sunroom with a half-round end on the 12 ft side | 176.55 sq ft |
| Six wall lengths, all right angles, no dimensions given | 160.375 sq ft |
| 1,850 sq ft in square meters | 171.87 m² |
| 3.65 m × 4.80 m room in square feet | 188.58 sq ft |
| Three rooms with feet and inches, total | 501.09 sq ft |
Each model saw every problem three times in two ways: once asked to show its working and end with a line reading ANSWER: <number>, and once told to reply with the number only. We then typed three of the problems the way people do ("how many sq ft is a 12'7" x 15'3" room"), three runs each. That makes 69 replies per model.
We scored percentage error against the exact answer, and called a reply an exact match if it was right to the precision the model chose: 192 and 191.9 both count for 191.896. Answers given as a fraction or in terms of π were evaluated, counted as correct, and logged as a format miss.
The results
342 of 345 replies were exact matches. One of 345 was blank, and two were rounded slightly wrong. The mean error was under 0.01% for every model. Both Claude models and gemini-3.1-pro-low went 69 for 69; gemini-3.8-flash-high got 68 and deepseek-flash 67. Each model's single worst answer was 0.03% off for claude-opus-5-5, gemini-3.1-pro-low and deepseek-flash, 0.06% for claude-haiku-5-5 and 0.26% for gemini-3.8-flash-high.
Each model's worst answer. The largest, 0.26%, was gemini-3.8-flash-high rounding a 176.55 sq ft sunroom to 177 sq ft; deepseek-flash's one empty reply has no % error and is not plotted. Source: ThreatFrontier test, 10 October 2026.
The trap we expected never sprang. The classic slip is to read 12' 7" as 12.7 ft, which gives 12.7 × 15.3 = 194.31 sq ft, 1.3% too high. Not one of the 345 replies did it. Every reply that showed its conversion either divided the inches by 12 or worked in inches (151 × 183 = 27,633 sq in) and divided by 144.
Every model answered the L-shaped room, the closet notch and the mixed-units room correctly in every run. The worst number in the test was gemini-3.8-flash-high rounding the sunroom to 177 sq ft against an exact 176.55: 0.26%, or under half a square foot.
Where they slipped
Right answer, wrong form. 36 of 300 replies in the first two rounds ignored the request for a plain number. gemini-3.1-pro-low answered the bedroom as 9211/48 in all six runs, and the sunroom in terms of π (120+18\pi) in all six. Both are exactly right, and both would break a spreadsheet cell or any script that expects a number. gemini-3.8-flash-high and deepseek-flash did the same six times each. The Claude models never did.
An answer that never came. On the six-wall room in number-only mode, deepseek-flash spent all 16,000 tokens on hidden reasoning and returned an empty reply. Its reasoning trace, which the API returned alongside, contains the right answer, 160.375, 37 times. A higher cap might have let it finish; as it was, the user got nothing.
Rounding too early. In their working, all six Gemini replies to the casually typed bedroom rounded 7 inches to 0.583 ft before multiplying and wrote down the result, 191.89 sq ft rather than 191.90. Six Claude and DeepSeek replies took the same shortcut but rounded the result to 191.9. One deepseek-flash reply gave the sunroom as 176.6 sq ft, though 176.549 rounds to 176.5. These are differences of a few hundredths of a square foot.
Working that doesn't add up. One claude-haiku-5-5 reply put the third room of the three-room problem at 243.951 sq ft; it is 243.88. Its stated total, 501.09, was still right, which means the final line did not come from the numbers above it. Another Haiku reply described 160.375 sq ft as "160 ft² 4½ in²". But 0.375 sq ft is 54 sq in; 4½ in is what 0.375 of a foot is, a length, not an area. The habit worth keeping: check the total against the steps, not just the total.
Advice you didn't ask for. For the casually typed bedroom, 10 of 15 replies suggested adding up to 10% for cutting waste and buying as much as 211 sq ft. That is reasonable advice for flooring, but it is a different number from the room's area, and it is easy to copy the wrong one.
How to calculate square feet of a room yourself
The method the models used is the one to use by hand:
- Split the room into simple shapes. An L is two rectangles. A notch is a rectangle you subtract. A bay is a trapezoid: average the two widths and multiply by the depth. A half-round end is π × radius² ÷ 2.
- Turn inches into decimal feet by dividing by 12: 7 in is 0.583 ft, 3 in is 0.25 ft. So 12' 7" × 15' 3" is 12.5833 × 15.25 = 191.90 sq ft. Or multiply in inches and divide by 144.
- Use one unit system per room. Convert meters to feet first (divide by 0.3048) or feet to meters.
- Convert area at the end. The international foot is exactly 0.3048 m, so 1 sq ft is exactly 0.09290304 m². A 1,850 sq ft house is 171.87 m². FloorGen has a page for converting square feet to square meters that uses this exact factor.
For rectangular rooms, FloorGen's free square footage calculator does steps 2 to 4. You enter each room's length and width in feet and inches or in meters, up to 30 rooms, and it shows each room's area in the units you chose and the total in both square feet and square meters. It handles rectangles only: an L-shaped room goes in as two rectangles, and a bay or half-round end you work out by hand with the formulas above. All rooms share one unit system, so a room measured in both meters and feet needs converting first.
The bigger error is which square footage
A model can multiply perfectly and still answer the wrong question. A house's quoted square footage is usually measured to the outside of its walls. Fannie Mae requires appraisers to follow the ANSI Z765-2021 standard, and North Carolina's real estate commission, comparing its own rules with that standard, tells brokers to "measure exterior walls from outside". Flooring, paint and furniture need the area inside the walls, and the gap between the two is the walls themselves.
FloorGen publishes a 1,500 sq ft house plan that shows the size of that gap. The house is 50 × 30 ft to the outside faces, exactly 1,500 sq ft. The 13 rooms, closets and halls in its room table, measured to the inside faces of the walls, add up to 1,339.3 sq ft. The other 160.7 sq ft, about 11% of the quoted figure, is wall. Ask a model to work out flooring from a listing's square footage and it will happily give you a precise answer to the wrong area.
FloorGen's 1,500 sq ft starter plan. Each label is a net area inside the walls; with the laundry and walk-in closet, which the render leaves unlabeled, the 13 rooms add up to 1,339.3 sq ft.
What FloorGen doesn't do
The calculator handles only rectangles, as above. In the floor-plan editor, every wall is a straight segment between two points, so a half-round sunroom can only be approximated with short straight walls. FloorGen doesn't measure anything for you: there is no LiDAR scanning, so the area is only as good as your tape. The areas it shows on rooms are net areas, inside the walls. That is what you want for flooring, but it is not the outside-of-walls finished area used in listings and appraisals.
Limits of this test
These were clean, fully specified problems. Real rooms are out of square, and measuring error will usually be larger than anything the models got wrong here. We tested through one gateway on one day, three runs per problem; replies to gemini-3.8-flash-high came back labeled with two different backend model names, so that id may not map to a single fixed model. The prompts, raw replies and scoring scripts are kept with this article, and a different token cap could change the one empty reply.
Disclosure: FloorGen is built by ThreatFrontier's publisher. FloorGen details were checked against its code and live site in October 2026.
Sources
- NIST, U.S. survey foot: revised unit conversion factors (international foot = 0.3048 m exactly): https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors
- FloorGen square footage calculator: https://floorgen.app/tools/square-footage
- FloorGen square feet to square meters converter: https://floorgen.app/tools/sqft-to-m2
- FloorGen 1,500 sq ft house plan (net room areas): https://floorgen.app/floor-plans/1500-sq-ft-house
- Fannie Mae Selling Guide B4-1.3-05, Improvements section of the appraisal report (ANSI Z765-2021): https://selling-guide.fanniemae.com/sel/b4-1.3-05/improvements-section-appraisal-report
- North Carolina Real Estate Commission, Residential square footage guidelines vs. ANSI standard: https://bulletins.ncrec.gov/ncrec-residential-square-footage-guidelines-vs-ansi-standard/