Calculation method & assumptions
Every number on SpecCalcs is computed at build time from the governing standard's formulas and constants — nothing is copied from third-party charts. The same engine renders the static tables and drives the interactive calculators, so the two can never disagree.
Bolt torque
Torque uses the industry short-form relation T = K·D·F with preload F = u·Sp·At at a default utilisation of 75 % of proof load. Proof strengths come from SAE J429 (unified grades) and ISO 898-1 / ISO 3506-1 (metric classes and A2 stainless). Tensile stress areas follow ASME B1.1 (inch) and ISO 898-1 (metric). Nut factors K are published mid-range values with their realistic spread shown alongside every result.
Known limits: K is empirical and swings with surface condition; achieved preload under torque control commonly scatters ±25 %. For safety-critical joints use angle-control or direct-tension methods and the fastener manufacturer's data.
Tap drill
Drill sizes use the machinist relation d = D − (%thread/76.98)·P. At the default 75 % engagement the published chart value is shown directly, so the calculator and the printed tables always agree. Thread dimensions follow ASME B1.1 and ISO 724.
Clearance hole
Clearance holes are standard table values, not a formula: metric from ISO 273 (fine / medium / coarse series) and inch from ASME B18.2.8 (close / normal / loose fit). Because clearance depends only on nominal diameter, one page covers every thread pitch of that size. Two inch sizes (#12 and 9/16) are left out where we could not confirm the standard's value against a reliable source — we would rather omit a size than publish a number we can't stand behind.
Metal weight
Weight is cross-section geometry × length × density. Densities are standard handbook values (carbon steel 7.85, stainless 8.00, aluminium 6061 2.70 g/cm³, etc.); real mill stock varies a few percent with alloy and tolerance.
Chain drive
Ratio, chain length and exact centre distance use the classic sprocket relations for ANSI/ASME B29.1 roller chain, with chain length rounded up to an even number of pitches before the centre distance is recomputed.
Cutting speed & RPM
Spindle speed comes from the uncontested relation n = SFM × 12 / (π·D). The surface-speed (SFM) bands per material are treated like the torque nut factor — a range, not a single number — because real cutting speed swings with tool coating, coolant and rigidity. Bands follow Machinery’s Handbook and tool-maker charts as turning baselines; milling and drilling are scaled by rule-of-thumb factors (≈0.8× and ≈0.6×) rather than tabulated per cell. Treat every value as a starting point to refine against your cutter’s own data.
Sheet metal — gauge & bending
Gauge-to-thickness is an exact table lookup, not a formula: steel from the Manufacturers’ Standard Gauge, stainless from the stainless gauge standard, aluminium from Brown & Sharpe. The same gauge number is a different thickness in each, so the pages are per material. Bend geometry uses the standard press-brake relations — BA = θ(R + K·T), BD = 2(R+T)·tan(θ/2) − BA — with the K-factor treated as a material-dependent range and starting default, since it shifts with radius, temper and bending method. For production, confirm K against a test bend.
Verification
Every build re-runs an automated test suite that checks computed values against published reference figures before any page is generated. If a check fails, the site does not build.
Found a value that looks wrong? The governing standard wins — check it first, then let us know so we can correct the engine.