How to measure square footage from a plan set

Which sheets carry the flooring

Open the set and pull four things. The floor plans are the geometry — one per level, usually the A-series, with dimension strings outside and room names inside. The enlarged plans for kitchens, baths and stairs are drawn larger, because that is where the small deductions live. The finish schedule says which room gets which floor, so it decides which rooms you measure. The room finish legend decodes its abbreviations — WD1, TL2, CPT — so the takeoff groups by material.

On a remodel, add the demolition plan, which says what is being pulled up. Elevations, details and reflected ceiling plans carry no floor area.

Each sheet states its scale near the title block — 1/4" = 1'-0" residential, 1/8" on larger buildings, 1/2" or larger on enlarged plans. Read it to know which sheet you are on, and use it for arithmetic only after the calibration below confirms it.

Dimension strings, and what they measure to

A dimension string is the chain of figures running along the outside of the plan: an overall dimension, a second breaking it into bays or wall-to-wall runs, and a third locating openings. Where a room has a complete string on two sides, its area is arithmetic.

The catch is what the string measures to. Architectural strings commonly run to the face or the centreline of the stud, because that is what a framer builds from. Flooring goes down inside the finished room, so the area you want is the net one: inside face of finished wall to inside face of finished wall. A room dimensioned stud face to stud face reads about an inch wider than the floor that fits in it — a square foot across a 15-foot room, a carton across twelve rooms.

Gross area, taken to wall centrelines, belongs to the architect's area schedule. Flooring comes from net area, which is most of any two or three percent disagreement between the two.

What comes out of the area

The floor is what is left once the fixed objects come out. Base cabinets and islands sit on the subfloor, so their footprints come out; the toe kick is a void under the cabinet. Tubs, shower pans, built-in seats and hearths come out. The stair opening comes out of the floor above, and the stairs are counted as treads and risers on their own line.

Closets are floored on most residential jobs and belong in the area. Where the same material continues through a doorway, carry the floor to the centre of the opening; where it changes, stop at the threshold and add the transition to the accessories list. Decide both once and write the decision on the takeoff.

Keep the deductions as their own lines, so a questioned number reads back off the takeoff.

Three ways to get the number

When each one is right

Dimension strings win on a small, rectangular, fully dimensioned job. They stop working the moment a room has an angled wall, a bay, or a string that skips what you need.

Calibrate and trace is the general case. It reaches the rooms the strings do not, keeps each measurement attached to its room, and leaves an outline behind — which is what a laying pattern and a render are drawn on later.

On-site verification answers a different question: what is there. For a bid, the plan set is the right source, because it is what every bidder prices against. Before ordering on a remodel, measure the building.

Calibrating a PDF sheet, precisely

A PDF carries geometry and a scale note, and the two come apart easily. The file is exported from CAD at one sheet size and re-exported at another. An 11x17 set is reduced onto letter for email. A printer applies fit to page and takes another few percent out. The geometry changes size at every step, and the title block still reads 1/4" = 1'-0", because it is text.

The printed dimensions change size along with everything else, and that is why they work: 14'-6" printed over a wall run is still 14 feet 6 inches of building however the sheet was scaled, so measuring that line gives the true ratio of file units to feet. SquareTally asks for this on every sheet — click one end of a printed dimension, click the other, and type the figure as it appears.

Four things make a calibration hold up:

• Pick the longest printed dimension on the sheet. A click three pixels off spreads its error across the length you calibrated on, so a 40-foot dimension divides that error by four against a 10-foot wall.
• Zoom in at both ends before clicking. The cost of a stray pixel falls as you magnify.
• Calibrate every sheet separately. Sheets are exported at different sizes, and enlarged plans are drawn at a different scale.
• Check it against a second printed dimension and see whether it reads back what it says. Two seconds, and it catches a misplaced click before twelve rooms are traced on it.

On a validated real plan, a room measured this way came to 152.6 sq ft against the 153 sq ft the architect annotated — 99.7%, well inside any waste allowance that follows. Re-calibrating recalculates every room already traced on the sheet.

Tracing rooms, angles and curves

With the scale set, a room is a closed outline: click each corner of the finished floor and close the shape. Trace to the inside face of the wall, and name each room the way the finish schedule names it, so the takeoff reads back against the schedule.

Angled walls are just more corners. A 45-degree cut-off corner, a splayed dining room, a bay window: click where the angle starts and where it ends, and the polygon handles the trigonometry.

Curves are approximated with chords. A radiused wall or a curved island is traced as a run of short straight segments. Segments drawn inside the arc report slightly less area than the curve; segments stepped outside report slightly more. A click every foot or so along the arc puts the difference well under a square foot. Where a curve is genuinely large, say so on the takeoff and let the installer template it on site.

Closets, niches and chimney breasts are the same move: a few extra corners. Trace the deductions as their own outlines too, because an outline can be checked later and a judgement call cannot.

A worked example

An open kitchen and dining area, one material throughout, laid straight.

Kitchen. The room measures 15'-4" by 12'-6" wall to wall: 15.333 × 12.5 = 191.67 sq ft gross. Base cabinets run 12'-0" along one wall and 8'-6" along another at a 2'-0" depth, so 12 × 2 = 24.00 and 8.5 × 2 = 17.00, and 41.00 sq ft comes out. The island is 7'-0" by 3'-6", another 24.50 out. Net kitchen: 191.67 − 41.00 − 24.50 = 126.17 sq ft.

Dining. The dining end is 11'-0" deep, 10'-0" on one side and 13'-0" on the other, with an angled wall between them — a trapezoid, so its area is the depth times the mean of the two sides: 11 × (10 + 13) ÷ 2 = 11 × 11.5 = 126.50 sq ft.

Threshold. The same floor runs to the centre of the 3'-0" opening into the hall, through a 5-inch jamb: 3 × 0.417 = 1.25 sq ft.

Measured total: 126.17 + 126.50 + 1.25 = 253.92 sq ft.

Waste goes on next, at the rate the laying style carries — 10% for a straight plank layout. 253.92 × 1.10 = 279.31, so 280 sq ft is the quantity to order in a material sold by the square foot.

Material sold by the carton rounds a second time. At 19.5 sq ft a carton, the standard for 3/4-inch by 2-1/4-inch solid strip oak, 279.31 ÷ 19.5 = 14.32, which is 15 cartons covering 292.5 sq ft. Against the 253.92 sq ft on the floor that is an effective waste of 15.2%. The square feet to boxes converter does that second rounding on its own.

The checklist to hand a new estimator

1. Pull the floor plans, the enlarged plans, the finish schedule and the legend.
2. Read the schedule first and mark which rooms get which material.
3. Calibrate each sheet on its own, including each enlarged plan, against the longest dimension printed on it.
4. Check that calibration against a second printed dimension before tracing anything.
5. Measure to the inside face of the finished wall.
6. Name every room the way the finish schedule names it.
7. Deduct base cabinets, islands, tubs, shower pans, hearths and the stair opening, each as its own line.
8. Count stairs as treads and risers, priced per piece, and include closets.
9. Carry the floor to the centre of a doorway where the material continues, and to the threshold where it changes.
10. Total per material and per laying pattern, then apply the waste rate belonging to each pattern.
11. Round to whole cartons last, and record the effective waste that rounding produced.
12. List anything the drawing left ambiguous and ask before the order goes in.