Built for the bench, not the boardroom
Most unit converters are written for spreadsheets. ConvertMM is written for sawdust and swarf. Every tool on this page came out of a real workshop question — the kind you mutter with a caliper in one hand and a bag of mystery bolts in the other. Is 11/16" bigger or smaller than 17mm? Does a 1/4-20 bolt take the same wrench as an M6? Which sheet-metal gauge is actually 2mm thick, and why does "gauge" mean something different for steel than for aluminium? These aren't abstract math problems. They're the five-second decisions that stand between a clean fit and a re-cut, a stripped thread and a solid joint.
If you live in the awkward border country between imperial and metric — fractional inches on your tape, millimetres on your drawings, and a drawer of fasteners in both systems — this is the reference you wish came bundled with your first tool kit. Nothing here needs a login, nothing nags you to upgrade, and nothing rounds off the digit that matters. The numbers are shown to the precision a machinist would actually work to, because being "close" is exactly how a drawer of parts stops fitting together.
From the tape measure to the lathe: what each tool actually solves
Start where most builds start — the tape. The Fraction-to-Decimal Inch Converter turns readings like 7/32" or 1-3/8" into clean decimals and millimetres, so you can drop them straight into a CAD sketch or a metric drill index without doing long division on the back of an offcut. It's the tool woodworkers open twenty times a day, and it quietly kills the number-one cause of bad cuts: misreading a crowded 16ths scale. Pair it with the Wire Gauge (AWG) to mm Converter when a project crosses into electrical work — AWG runs backwards (bigger number, thinner wire), and getting diameter and cross-sectional area in mm² right is the difference between a cable that carries the load and one that runs warm.
When you move to fasteners, the Thread Pitch & TPI Converter and the Bolt Grade & Tensile Strength Converter do the translating so you don't have to guess. One tells you whether that imperial threads-per-inch matches a metric pitch in millimetres; the other maps property classes like 8.8, 10.9 and 12.9 onto their SAE cousins and shows you the proof load you're actually working with. Torque them properly with the Torque Unit Converter, which swaps cleanly between Newton-metres, pound-feet, pound-inches and kgf·m — because a spec sheet that says "45 Nm" and a torque wrench marked in lb-ft is a recipe for an over-tightened, snapped-off morning.
Metalwork, sheet, pipe and the weight of it all
Fabricators get their own bench of tools. The Sheet Metal Gauge Thickness Converter is one of those references you shouldn't have to memorise and never will, because 16-gauge steel, aluminium and stainless are all genuinely different thicknesses — the converter shows exact inches and millimetres per material so your bend allowances and clearances stay honest. The Material Weight Calculator handles round bar, flat plate and hollow tube in any metal: feed it your dimensions, pick a material, and know the weight before you lift it, quote it, or trust a shelf bracket with it. For plumbing, hydraulics and structural tube, the Pipe Size NPS to mm Converter untangles the industry's oldest trick — that "1-inch" nominal pipe whose outside diameter is nothing like an inch — giving you real OD and wall thickness by schedule.
Two more tools cover the finish and the fluid. The Surface Roughness Ra Converter moves between Ra in micrometres and microinches, RMS, and ISO N-grade numbers, so a drawing callout of "N7" or "1.6 µm" actually means something at the grinder. And the Volume & Capacity Converter for Tanks calculates full and partial-fill volumes for cylindrical, rectangular and horizontal tanks in litres and US/UK gallons — useful whether you're sizing a compressor receiver, a dust-collector drum, or a water butt. Rounding out the set, the Pressure Unit Converter keeps PSI, Bar, kPa, MPa, atm and mmHg straight, and reminds you which way gauge and absolute pressure differ.
How to actually use these without getting burned
A converter is only as good as the habit around it, so a few hard-won rules. First, convert once and write it down — don't reconvert a converted number, because every extra rounding step drifts the value a little further from true, and across a whole build those drifts stack into the dreaded off-by-a-millimetre gap. Second, always carry the unit with the number: "3/8" and "3/8 mm" are wildly different holes, and the tape doesn't care which one you meant. Third, when a fastener's system is unknown, measure the pitch and diameter and let the thread and bolt-grade tools identify it rather than forcing an imperial wrench onto a metric head "because it nearly fits." Nearly-fits is how threads get chewed.
Everything here is deliberately built to be read at a glance and trusted at the bench — precise digits, no clutter, and no assumption that you're sitting at a desk. Bookmark the two or three tools your trade leans on, keep the tab open next to your cut list, and treat this like the printed conversion chart that used to live taped inside every good toolbox lid — except this one covers threads, gauges, torque and tank volumes too, and it's never out of date. Build twice, cut once, and let the arithmetic be the one thing you don't have to double-check.