Editorial
How these calculators are checked
Every formula on this site lives in a plain TypeScript function, separate from the page. A unit test feeds it a known case — a 1×12×1 board foot, a perfect bowling game, a 6/12 rafter on a 24-foot span, the published 100 kg / 600 kg DOTS score — and the test fails if the result moves. The page cites the same identity the test locks.
Rounding is part of the spec when the trade rounds. Quilt yardage goes up to the next eighth of a yard. IPF GL rounds the body-weight coefficient to six decimal places before multiplying, because that is what the score sheet does. Money is shown in cents; the underlying payment is not rounded until display, and the amortization test checks that 360 payments retire a 6% loan.
Fees are dated, not scraped
Seller rates are constants in the repository, with a last-verified date of 2026-10-17. Etsy, Whatnot, and PayPal were read from the platforms’ own pages. Stripe’s US online card rates are the figures recorded for this batch on the same date:
- Etsy Fees & Payments Policy — $0.20 listing, 6.5% transaction, offsite ads 15% / 12% with a $100 cap.
- Etsy Payments Policy — US processing 3% + $0.25 on the gross order, including shipping and tax.
- Whatnot seller fees — article updated 2026-10-01. US commission tiers and 2.9% + $0.30 processing.
- PayPal merchant fees — page date 2026-10-01, read 2026-10-17. Goods & Services is 2.99% with no fixed fee on that line. Checkout is 3.49% + $0.49. Cards are 2.99% + $0.49. International commercial payments add 1.50%.
- Stripe pricing — US online cards 2.9% + 30¢, international cards +1%, disputes $15. Currency conversion is separate and is not given a percent.
A regulatory operating fee applies to Etsy sellers in some countries and is not modeled; US shops are not charged it. Whatnot’s non-US rate cards differ and are not copied here. If a platform changes a number, the constant, the date, and the test should change together. The page shows the date so a stale rate is obvious.
What “reviewed” means
The checked date on a calculator is the day the formula, the units, and the worked example were walked against the source below. It is not a claim that a person with a trade license signed the page. There is no invented expert byline.
| Calculator | Source of the math |
|---|---|
| Cross stitch | Stitch count ÷ (fabric count ÷ threads per stitch), plus margin on both sides. Counted-thread measurement used by fabric shops. |
| Quilt size | Mattress plus a drop on both sides. Length adds one drop and the pillow tuck. Whole blocks; leftover ÷ 2 is the border. |
| Quilt border | Butted sides match the current length; ends add two border widths. Mitered strips add two border widths plus extra. Cut width adds ½ in. Yardage rounds up to ⅛ yard. |
| Half-square triangles | Two at a time: finished + ⅞ in. Four at a time: finished × √2 + 1¼ in, up to ⅛ in. Eight at a time: 2 × finished + 1¾ in. Geese: width + 1¼ in and height + ⅞ in. |
| Fabric yardage | floor(width ÷ piece) across, ceil(quantity ÷ across) rows, rows × length ÷ 36, up to ⅛ yard. Non-directional cloth keeps the cheaper orientation. |
| Blanket yarn | Area × a planning factor by Craft Yarn Council weight number. Crochet × 1.30. The Council names the weights; the yards per square inch are not a Council figure. |
| Knitting gauge | Cast-on = width × stitches per 4 in ÷ 4. Percent = (yours − pattern) ÷ pattern. |
| Increases and decreases | Flat rows put the remainder at the end. In the round the remainder is split. A decrease consumes two stitches. |
| Quilt backing | Overage added to each side, then the cheaper of the two piecing directions. Yardage rounds up to the next 1/8 yard. |
| Quilt binding | 2 × (width + length) + a join allowance, divided into width-of-fabric strips. |
| Resin | Geometry of the pour, then 128/231 US fl oz per cubic inch. Parts split by the stated volume ratio. |
| Candle | Water weight × specific gravity, then wax = fill ÷ (1 + fragrance load). Armatage Candle Company candle-math equations. |
| Sourdough | Baker’s percentages. Total flour, including flour in the starter, is 100%. |
| Board foot | (thickness in × width in × length ft) / 12. A 1×12×1 is one board foot. |
| Board and batten | n boards and n−1 gaps fitted to the wall, gap recomputed so the courses fill the width. |
| Rafter | Pythagorean common rafter. Run is half the span minus half the ridge thickness. |
| Stair stringer | ceil(rise ÷ max riser) equal risers, one fewer tread, hypotenuse of rise and run. IRC R311.7.5 riser and tread limits. Throat is stock minus the perpendicular notch. |
| Gravel, mulch, soil, stone | Shape × depth, ÷ 27 for yards. Ordering densities in tons per cubic yard, with waste and, for soil, a settle factor. Bag counts round up. |
| Cubic yard | 27 cubic feet. Rectangle, circle, triangle, or a known area. |
| Concrete | Same volume. Bags from Quikrete Concrete Mix 1101: 0.30 / 0.45 / 0.60 cu ft. Weight at 150 lb/ft³. Solid steps use n(n+1)/2. |
| Asphalt | Compacted 145 lb/ft³. Tons = cubic feet × 145 / 2,000. Depth is finished thickness. |
| Stair | Equal risers, one fewer tread per flight. IRC R311.7.3 flight rise 151 in. R311.7.6 landing 36 in. Cross-linked to the stringer cut. |
| Roof pitch | Pitch per 12, degrees = arctan(slope), percent = slope × 100, multiplier = √(1 + slope²). |
| Ballistic | Point-mass G1/G7, BRL Cd(Mach), |a| = ρ π g Cd v² / (1152 BC). Checked against a Shooter’s Calculator G7 chart. No reloading data. |
| Gear ratio | Nominal tire diameter. RPM = mph × axle × gear × 63360 / (60 π diameter). |
| Compression ratio | (swept + chamber) / chamber. Chamber includes gasket, deck, and a signed piston volume. Dynamic stroke at IVC. |
| Wheel offset | Backspace = (width + 1 in) / 2 + ET/25.4. Poke and inner clearance are the differences. |
| Wire size | NEC 2023 Table 310.16, 75°C column, plus 240.4(D) and the Chapter 9 voltage-drop K method. 3% is advisory. |
| ABV | (OG − FG) × 131.25, the alternate gravity formula, and Terrill’s 2011 refractometer cubic. |
| Conduit fill | NEC Chapter 9 Table 1 fill (53% / 31% / 40%, 60% for a nipple ≤ 24 in). Table 4 and Table 5 areas. Note 7 rounds a same-size count up at 0.8. |
| Voltage drop | 2 K I L / cmil single phase, √3 K I L / cmil three phase. K = 12.9 copper, 21.2 aluminum. Circular mils from Chapter 9 Table 8. 3% and 5% are informational notes. |
| RPE | Reactive Training Systems chart (Mike Tuchscherer). 5 reps at RPE 8 = 81.1% of 1RM. |
| DOTS, Wilks, IPF GL | Published polynomials and the IPF GL coefficient sheet (1 May 2020). GL coefficient rounded to 6 decimals. |
| Bowling | Ten-pin rules: strike adds the next two rolls, spare adds the next one, tenth frame may have fill balls. |
| Arrow | AMO front-of-center. Easton kinetic energy: grains × fps² / 450,240. Speed scales from the IBO 70 lb / 30 in / 350 gr condition. |
| Ski length | Retailer landmark chart: beginner chin to nose, intermediate nose to forehead, advanced head height and above. Not a DIN setting. |
| Snowboard length | All-mountain chin to nose, park 2 in shorter, powder longer. Wide board at a US men’s boot of 11 or larger. |
| Bullet energy | grains × fps² / 450,437. Momentum = grains × fps / 225,218. Joules = ft·lbf × 1.35582. Not the arrow page’s 450,240. |
| Crosswind | FAA crosswind geometry. Component = wind × sin(angle). Headwind = wind × cos(angle). |
| Mortgage | Standard fixed-rate annuity. M = P r (1+r)^n / ((1+r)^n − 1). |
| BMI | kg/m², or lb/in² × 703 (CDC). WHO adult cutoffs. Not a diagnosis. |
| Etsy | Listing 0.20 USD, transaction 6.5%, processing from the Etsy Payments table. Verified 2026-10-17. |
| Whatnot | US tier card effective 2026-09-21. Processing 2.9% + $0.30. Help article updated 2026-10-01. Verified 2026-10-17. |
| PayPal | Checkout 3.49% + $0.49. Goods & Services 2.99% and no fixed fee. Cards 2.99% + $0.49. International +1.50 points. Page date 2026-10-01. Verified 2026-10-17. |
| Stripe | US online cards 2.9% + $0.30. International cards +1 point. Dispute $15, optional. Recorded 2026-10-17. Currency conversion is not priced. |
Old questions
The previous site’s FAQ URL redirects here. If a result disagrees with a cited source, the source wins and the function should change.