Estimating math on the B exam
The B study guide says some questions require mathematical computation and that a calculator will be provided. None of it is hard. Most wrong answers come from a unit left unconverted.
You will not be asked for calculus. You will be asked to turn a plan dimension into a quantity someone can order: cubic yards of concrete, board feet of lumber, squares of roofing, a count of studs, a rafter length. The wrong choices are built from predictable slips: inches left as inches, a diameter used as a radius, markup confused with margin, a piece count one short. CSLB's own sample questions in the B study guide include a volume calculation.
The kinds of calculation
| What | How |
|---|---|
| Area (floors, walls, roofs, sheathing, paint) | length × width, in feet; triangles ½ × base × height; subtract openings only if the question says to |
| Volume (slabs, footings, excavation, fill) | length × width × depth, all in feet, then ÷ 27 for cubic yards |
| Round piers and pipes | π × radius² × length; the radius is half the diameter |
| Lumber | board feet = thickness (in) × width (in) × length (ft) ÷ 12, on nominal sizes |
| Roofs | plan area × slope factor = roof area; ÷ 100 = squares |
| Rafters and stairs | the right triangle: run² + rise² = length² |
| Layout counts (studs, joists, anchors) | run ÷ spacing, round up, + 1 |
| Waste and swell | quantity × (1 + the percentage the question gives) |
| Labor and price | hours × rate; markup is on cost, margin is on price |
The conversions to know cold
| Conversion | Value |
|---|---|
| Cubic feet in a cubic yard | 27 |
| Square feet in a square yard | 9 |
| Square inches in a square foot | 144 |
| Cubic inches in a cubic foot | 1,728 |
| Square feet in an acre | 43,560 |
| Gallons in a cubic foot | about 7.48 |
| Cubic inches in a gallon | 231 |
| Square feet in a roofing square | 100 |
NIST Handbook 44 (2026), Appendix C, tables of units of area and volume. Roofing square: DOE Building America Solution Center (drainage planes: “one square = 100 square feet”) and National Park Service Preservation Brief 29.
Roof slope is written as rise in 12 inches of run: a 6:12 roof rises 6 inches per foot. The slope factor that turns plan area into roof area is √(rise² + 144) ÷ 12. For 4:12 it is about 1.054, for 6:12 about 1.118, for 8:12 about 1.202, for 12:12 about 1.414. You do not need to memorize the table if you can do the square root on the calculator.
Example 1: a slab, in cubic yards
A garage slab is 24 ft by 36 ft and 4 inches thick. The question tells you to allow 5% for waste. How many cubic yards do you order?
Thickness in feet: 4 ÷ 12 = 0.333 ft. Volume: 24 × 36 × 0.333 = 288 cu ft. Cubic yards: 288 ÷ 27 = 10.67. With waste: 10.67 × 1.05 = 11.2 cubic yards.
The trap: 24 × 36 × 4 = 3,456, ÷ 27 = 128 — the inches never became feet. An answer ten times too big is usually this.
Example 2: board feet
A lumber order is 40 pieces of 2×6 at 12 ft and 10 pieces of 4×12 at 16 ft. How many board feet is that?
2×6×12: 2 × 6 × 12 ÷ 12 = 12 board feet each, × 40 = 480. 4×12×16: 4 × 12 × 16 ÷ 12 = 64 each, × 10 = 640. Total 1,120 board feet, or 1.12 MBF (thousand board feet). If the question prices lumber per MBF, multiply 1.12 by that price.
The trap: using the actual dry size (1½ × 5½) instead of the nominal 2 × 6. Board feet are figured on nominal sizes, which is also how Caltrans pays for timber by the thousand foot board measure (Standard Specifications 2025, §57-1.04). A board foot is a piece 1 inch thick, 12 inches wide and 1 foot long.
Example 3: rafter length and roofing squares
A gable house is 28 ft wide and 40 ft long with a 6:12 roof and an 18-inch overhang, measured level, on every side. Find the rafter length from ridge to tail, and the roofing squares with 10% waste.
Run to the ridge: 28 ÷ 2 = 14 ft. Rise: 14 × 6 ÷ 12 = 7 ft. Rafter to the wall line: √(14² + 7²) = √245 = 15.65 ft. The overhang adds 1.5 × 1.118 = 1.68 ft along the slope. Total about 17.3 ft, so you would buy 18-ft stock. (Framers also deduct half the ridge board thickness; do that only when the question gives a ridge size.)
Roof area: the plan including overhangs is 31 × 43 = 1,333 sq ft. × 1.118 = 1,490 sq ft of roof. ÷ 100 = 14.9 squares. With 10% waste: 14.9 × 1.10 = 16.4 squares.
The traps: using the building footprint without the overhangs, and forgetting the slope factor, which makes the roof look 11% smaller than it is.
Example 4: a wall, from studs to labor
A 36-ft exterior wall is framed at 16 inches on center, with a single bottom plate and a double top plate. The question says the crew frames 12 lineal feet of wall per hour and labor costs $58 an hour for the crew. How many studs (ignoring corners and openings), how many feet of plate, and what labor cost?
Studs: 36 ft = 432 in. 432 ÷ 16 = 27 spaces. Add one for the end: 28 studs. Plates: 3 × 36 = 108 lineal feet. Labor: 36 ÷ 12 = 3 hours × $58 = $174.
The trap: 27 studs. The number of spaces is not the number of pieces. The same “+ 1” applies to joists, rafters and anchor bolts.
Markup, margin and the other money math
Planning and Estimating includes cost estimation, and price questions tend to set the same trap as on the Law and Business exam: markup is a percentage of cost, margin is a percentage of the selling price. A $10,000 job marked up 25% sells for $12,500; the $2,500 profit is 20% of the price, not 25%. To make 8% of the price on a $46,000 cost, divide by 0.92: $50,000. Adding 8% to cost gives $49,680, which falls short.
Using the test-center calculator
- You cannot bring your own. Personal calculators are on the list of items not allowed in the testing room (CSLB Examinations FAQ).
- Write the units next to every number on your scratch work. That is what catches the inches-as-feet mistake.
- Estimate before you calculate. A 24 × 36 slab 4 inches thick is roughly 10 yards; if the calculator says 128, something is off.
- Round only at the end, and round up for anything you order.
Common questions
Is there math on the B General Building exam?
Yes. The study guide says some questions require computation and a calculator will be provided.
How do you convert cubic feet to cubic yards?
Divide by 27 (3 × 3 × 3). Convert every dimension to feet first: a 4-inch slab is 0.333 ft thick.
How do you calculate board feet?
Nominal thickness (in) × nominal width (in) × length (ft) ÷ 12. A 12-ft 2×6 is 12 board feet.
Last reviewed 29 September 2026. Conversions from NIST Handbook 44 (2026), Appendix C; roofing square from the DOE Building America Solution Center and NPS Preservation Brief 29; nominal lumber measure from Caltrans Standard Specifications 2025, §57-1.04; exam format from the CSLB General Building (B) study guide 13E-B (03-2026) and the CSLB Examinations FAQ. The worked examples are ours, not exam questions. We are not affiliated with CSLB or PSI.