Foster Tools

How we calculate

Foster Tools is built and maintained by Foster Apps LLC, a small independent software studio. We build software. We aren’t contractors or engineers, so we don’t invent numbers. Here is what we do instead:

  • Published formulas and values. Each tool uses standard formulas and reference figures. They’re listed below with their sources, and anything that’s our own planning assumption is labelled as one.
  • Show the math. Every result has “How this was worked out”, which runs your numbers through each step so you can check them.
  • One codebase, tested. The web calculators run the same code as our iPhone apps, and automated tests check each tool against worked examples before anything ships.
  • Fix mistakes in the open. If a result looks wrong, use “Report a problem” under it or email support@fosterapps.com. We correct the tool and update this page.

Results are estimates for planning. Check them against the codes, manufacturer data and supplier figures for your job, and have a qualified professional review work that needs one. See the Terms of Use.

Duct size calculator

Duct area = CFM ÷ velocity (FPM); round diameter = √(4 × area ÷ π). Rectangular sizes use the equivalent-diameter formula 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Friction rate uses the Darcy-Weisbach equation with the Altshul-Tsal friction factor for standard air at 70°F.

Assumptions

  • Velocity targets of 500, 700 and 900 FPM for quiet, standard and high-capacity runs are common residential rules of thumb, not a Manual D design.
  • Duct roughness: 0.0003 ft for galvanized steel; flex, PVC and fiberboard use published approximations. Real flex duct loses more when it's compressed or sagging.
  • Standard round sizes are the common off-the-shelf diameters from 3" to 48".

Sources

Roof pitch calculator

Pitch is rise per 12 of run: rise ÷ run × 12. Angle = arctan(rise ÷ 12); grade = rise ÷ 12 × 100; pitch factor = 1 ÷ cos(angle). Roof area = footprint × pitch factor, plus waste; a square is 100 ft².

Assumptions

  • The 2/12 warning follows the International Residential Code, section R905.2.2, which sets 2:12 as the minimum slope for asphalt shingles (with double underlayment up to 4:12). Your local code may differ.
  • Bundles assume 3 bundles per square, the most common packaging. Check the wrapper, since some shingles come 4 to a square.
  • Measuring with the phone reads its tilt sensor, so lay it flat on the rafter or roof surface and use Hold when the reading settles.

Sources

  • International Code Council, International Residential Code, section R905.2.2 (asphalt shingle slope; printed code)

Conduit bending calculator

Distance between bends = offset height × multiplier (1 ÷ sin of the angle); shrink = offset height × tan of half the angle. A 3-point saddle uses a 45° center and two 22.5° outer bends, with the outer marks 2.5 × the height from the center; add one half's shrink (3/16" per inch of height on bender charts) to the center-mark measurement, and the whole run shortens by twice that; a rolling offset bends to the true offset √(rise² + roll²); a 90° stub-up marks stub height − bender take-up.

Assumptions

  • Bender chart mode uses the rounded multipliers and shrink printed on bender charts (10°: 6 and 1/16"; 22.5°: 2.6 and 3/16"; 30°: 2 and 1/4"; 45°: 1.4 and 3/8"; 60°: 1.2 and 1/2") and 2.5 for a 3-point saddle. Exact mode uses the trigonometry instead.
  • Take-up (deduct) values are typical for hand benders. They differ between makes, especially above 1" (for example, Gardner Bender lists 12" for 1-1/4" EMT where this table shows 11"). Use the number stamped on your bender.
  • Results are layout geometry. They don't account for springback or the bender's radius beyond the take-up.

Sources

Asphalt calculator

Tons = length × width × compacted depth (ft) × 145 lb/ft³ ÷ 2,000, plus waste, rounded up to a quarter ton. Metric uses 2.32 t/m³, the same density.

Assumptions

  • 145 lb/ft³ for compacted hot mix. Real mixes run about 144–160 lb/ft³ depending on aggregate and compaction; your plant's mix design (JMF) has the exact figure.
  • Waste defaults to 10%. Crews typically add 5–10% for edges, spillage and an uneven base.
  • Truck loads use typical capacities of 7, 14 and 20 tons for single, tandem and tri-axle trucks. Legal loads vary by state and truck, so check with your hauler.

Sources

Driveway sealer calculator

Gallons = area ÷ coverage per gallon × coats, rounded up to a whole gallon, then to 5-gallon pails. With concentrate, the water is added on top of the concentrate at your mix percentage.

Assumptions

  • Coverage starts at 100 ft² per gallon of mixed sealer per coat for spraying. SealMaster quotes 100–120 ft² per gallon of concentrate once mixed, which is roughly 80–105 ft² per mixed gallon at 15–25% water, so 100 sits near the top of that range.
  • Squeegee or brush (75 ft²/gal) and the surface factors (× 0.8 average, × 0.6 rough or porous) are planning assumptions, not a manufacturer figure. Your product's label rate is the one to trust; Pro lets you enter it.
  • SealMaster recommends two coats of mixed sealer for best results.

Sources

Paint calculator

Paintable area = wall length × height − openings. Gallons = area × coats ÷ coverage per gallon, rounded up to a whole gallon (or a quart with Pro).

Assumptions

  • Coverage per gallon by finish: flat 400, eggshell 380, satin 375, semi-gloss 350, high gloss 325 ft². Manufacturers quote about 350–400 ft² per gallon for typical paint; glossier finishes spread a little thinner.
  • A door counts as 21 ft² (3 × 7 ft) and a window as 15 ft² (3 × 5 ft).
  • Pro options: textured walls cover 30% less, ceilings 5% more, trim 20% less; a dark-to-light change adds 30% paint; primer covers 300 ft² per gallon.

Sources

Baker’s percentage calculator

Every ingredient is a percentage of total flour, and total flour is 100%. The starter, poolish or biga is split into its own flour and water by its hydration (a 100% starter is half flour, half water), and those are added to the dough's flour and water before the percentages are worked out.

Assumptions

  • Starter and preferment hydration default to 100% (equal weights of flour and water). Change it if yours is stiffer or wetter.
  • The salt warning uses 1.5–2.5% of flour, the usual range for bread.

Sources