Four measurements come up in almost every workshop job, and all four are arithmetic rather than judgement: how much timber you are buying, what hole a thread needs, what the thread is called, and how far apart the fixings go.
Each has one rule worth carrying in your head, and each has a failure mode that only shows up after the material is cut. Getting them right on paper costs a minute.
How is a board foot measured?
As a volume: one foot by one foot by one inch, which comes to 144 cubic inches. Anything with the same volume is the same number of board feet whatever its shape.
The working formula multiplies thickness in inches by width in inches by length in inches and divides by 144. With the length in feet the divisor becomes twelve, which is the version most people memorise.
The important convention is that board feet are calculated on the nominal dimensions the timber is sold under rather than the finished ones it actually measures — a distinction the nominal sizes article covers in full. Calculating on the planed size undercounts what you paid for.
What size drill does a tap need?
One roughly the major diameter minus the pitch, which produces a thread engaging around three quarters of its theoretical depth.
| Thread | Pitch | Tapping drill |
|---|---|---|
| M4 | 0.7 mm | 3.3 mm |
| M5 | 0.8 mm | 4.2 mm |
| M6 | 1.0 mm | 5.0 mm |
| M8 | 1.25 mm | 6.8 mm |
| M10 | 1.5 mm | 8.5 mm |
Cutting a fuller thread is possible and rarely worth it. Going from seventy-five per cent to full engagement adds only a few per cent of strength and makes tapping dramatically harder, which is how taps get broken in a hole.
A clearance hole is a different calculation entirely — it has to let the fastener pass, so it is slightly larger than the major diameter rather than smaller.
What does M8 by 1.25 mean?
A metric thread with an 8 millimetre major diameter and 1.25 millimetres between adjacent thread crests. The pitch is the distance from one thread to the next, measured along the axis.
Each diameter has a standard coarse pitch, and when the pitch is omitted that is what is meant — so M8 alone means M8 by 1.25. Fine pitches exist for the same diameters and are specified explicitly because they are the exception.
Mixing them is possible for the first turn or two and destroys both parts. A fine-pitch nut will start on a coarse bolt of the same diameter and jam, which is why the pitch matters as much as the size when buying.
How far apart should fixings go?
Close enough to share the load and far enough from the edge not to split the material. The edge distance is the constraint people underestimate — a screw too close to the end of a board splits it along the grain regardless of how carefully it was driven.
Spacing along a run is a function of the load and the stiffness of what is being fixed. A stiff rail spreads load between fixings and tolerates wider spacing; a thin one does not, and sags between them.
For anything structural the spacing comes from a specification rather than a rule of thumb. For shelving, cladding and general joinery, an even spacing calculated to land the end fixings at a sensible distance from the edges is what matters most.
Why calculate rather than measure as you go?
Because the errors accumulate. Marking each fixing from the last one carries every small error forward, so the last one in a run of twelve can be several millimetres out.
Working out the spacing first and measuring every position from one end keeps each mark independent. It is the same reason a cutting list is worth writing before cutting rather than after each piece.
It also catches the impossible plan early — a spacing that does not divide the run evenly, or a fixing that would land on a joint, is easier to fix on paper.
What about sheet materials?
They are sold by area rather than by volume, and their nominal thickness is closer to the real one — a sheet described as 18 millimetres is usually within a fraction of it, because it is manufactured to thickness rather than sawn and dried.
The dimension that catches people out there is the sheet size itself, since the two common standards are near enough to look interchangeable and different enough to ruin a cutting plan. Checking the actual sheet before laying out cuts costs a minute.
Questions people ask
Is nominal size the same everywhere? The convention is, and the exact finished dimensions vary slightly by supplier and by moisture content.
Do I need a pilot hole? In hardwood and near edges, always. In softwood away from an edge, often not for small screws.
What is the difference between pitch and threads per inch? Two ways of stating the same thing. Metric quotes the distance, imperial quotes the count.
Does timber shrink after I buy it? It moves with humidity throughout its life, mostly across the grain rather than along it.
Work it out before cutting. The board foot calculator prices timber by volume, the drill size calculator gives the tapping and clearance holes, the metric thread calculator covers the thread specification, and the screw spacing calculator divides a run evenly with sensible end distances.