12 Measurement Conversions Every Woodworker Should Memorize

I've been woodworking long enough to have ruined a $40 piece of walnut because I misread millimeters as inches on a spec sheet. It happens. European hardware, Japanese chisels, imported sheet goods — the shop floor is a collision zone of measurement systems, and if you're fumbling with a conversion app every five minutes, you're losing time and making mistakes.

So here are the twelve conversions I actually use, written down the way my brain files them — not as a dry table, but as the kind of thing you'd scrawl on a shop apron pocket and memorize over a few months of real work.

1. The One That Saves You Most Often: 1 Inch = 25.4 mm

Burn this one in. Everything else flows from it. A 3/4-inch plywood top is 19.05 mm. A 1/4-inch router bit shank is 6.35 mm. If someone gives you a dimension in millimeters, divide by 25.4 and you're in inches. Round to the nearest 1/64 and move on.

Quick mental shortcut: 25 mm ≈ 1 inch (within half a millimeter). Good enough for joinery cuts, not precise enough for precision fitting.

2. Fractional Inches to Decimals: The Core Four

Your tape measure is fractional. Your calculator is decimal. These four are worth memorizing cold:

1/8" = 0.125"
1/4" = 0.250"
3/8" = 0.375"
1/2" = 0.500"
5/8" = 0.625"
3/4" = 0.750"
7/8" = 0.875"

Yes, that's seven — I know I said four. But once you see the pattern (numerator divided by 8, repeated in 0.125 steps), these just stick. The annoying ones are the sixteenths: 3/16" = 0.1875", 5/16" = 0.3125". I still occasionally look those up.

3. Board Foot Formula: (Thickness × Width × Length) ÷ 144

Every hardwood dealer sells by the board foot, and every woodworker eventually gets burned by not knowing how many they actually need. A board foot is 144 cubic inches — picture a board 1 inch thick, 12 inches wide, and 12 inches long.

The formula: multiply thickness (in inches) × width (in inches) × length (in inches), then divide by 144.

Example: An 8/4 walnut slab (2" thick, 10" wide, 96" long): 2 × 10 × 96 = 1,920 ÷ 144 = 13.3 board feet. Always add 20–25% for waste, defects, and that one cut you'll mess up.

4. Lumber Thickness: Nominal vs. Actual

A "2x4" is not 2 inches by 4 inches. It's 1.5" × 3.5". This trips up beginners building their first workbench more than almost anything else. Here's the dimensional lumber reality check:

1×4 = 0.75" × 3.5"
1×6 = 0.75" × 5.5"
2×4 = 1.5" × 3.5"
2×6 = 1.5" × 5.5"
2×8 = 1.5" × 7.25"
4×4 = 3.5" × 3.5"

The pattern: boards under 2" nominal lose 1/4", boards 2" and above lose 1/2". Widths: 3" or 4" nominal loses 1/2", everything wider loses 3/4". Ugly but consistent.

5. Hardwood Thickness: The Quarters System

Hardwood is sold in quarter-inch increments called "quarters." 4/4 means four quarters of an inch thick — nominally 1 inch, but it arrives at the yard surfaced to 13/16" or 7/8". Here's what you'll actually measure after surfacing:

4/4 (nominal 1") → ~13/16" to 7/8" surfaced
5/4 (nominal 1.25") → ~1-1/16" surfaced
6/4 (nominal 1.5") → ~1-5/16" surfaced
8/4 (nominal 2") → ~1-3/4" surfaced
12/4 (nominal 3") → ~2-3/4" surfaced

If you need a 1.5-inch table leg, you're milling 6/4 stock — not 8/4 unless you want to waste a lot of wood.

6. Sheet Goods: What "4x8" Actually Means

A 4×8 sheet of plywood is nominally 48" × 96". In practice it's usually 47-3/4" × 95-3/4" because of sanding at the mill. Plan your cuts accordingly — you can't always get four 12-inch rips from a 48-inch sheet if you're banking on full nominal width.

Plywood thickness nominal vs. actual is its own adventure:

1/4" plywood → typically 7/32" (about 5.5 mm)
1/2" plywood → typically 15/32" (about 12 mm)
3/4" plywood → typically 23/32" (about 18 mm)

This matters hugely for dado cuts and cabinet box construction. Always measure your actual sheet before setting your router table or table saw.

7. Metric Screw Sizes to Imperial Equivalent

IKEA flatpack hardware, imported cabinet hinges, European drawer slides — they all use metric screws. Here's the cross-reference you'll reach for most:

M3 ≈ #4 screw (2.5 mm shaft)
M4 ≈ #8 screw (3.2 mm shaft)
M5 ≈ #10 screw (4 mm shaft)
M6 ≈ 1/4" bolt (4.8 mm shaft)
M8 ≈ 5/16" bolt (6.4 mm shaft)

These aren't exact swaps — thread pitch differs — but for wooden assemblies where you're mostly concerned with shaft diameter and pilot hole sizing, these approximations work in a pinch.

8. Drill Bit Sizes: Fractional to Metric

This one bites when you're working from a European plan that calls for a 6 mm pilot hole and your bit set is fractional. The conversion: mm ÷ 25.4 = inches.

Most-used in the shop:

3 mm = 1/8"
4 mm ≈ 5/32" (close enough; 4.0 vs. 3.97)
5 mm ≈ 13/64" (or use 3/16" and go slightly undersized)
6 mm ≈ 15/64" (1/4" is 6.35 mm — nearly the same)
8 mm ≈ 5/16"
10 mm ≈ 25/64" (or just use a 3/8" and expect a slightly loose fit)

9. Degrees to Saw Blade Tilts: The Complements That Matter

Most miter gauges and bevel angles are set in degrees. But when you're cutting a compound miter or a box joint at an odd angle, you need to know the complement (90° minus your angle) because your saw references from 90°, not 0°.

Common conversions:

A 45° miter cut = set bevel to 45°
A 30° bevel = tilt blade to 30° (complement is 60°)
For 4-sided boxes with slanted sides: top angle ÷ 2 = blade bevel angle
Pentagon: 54° bevel. Hexagon: 60° bevel. Octagon: 67.5° bevel.

The octagon one is the only weird one. 360° ÷ 8 sides ÷ 2 = 22.5° from vertical = 67.5° blade tilt. I still check this every time.

10. Running Feet to Linear Feet to Square Feet

These terms get mixed up at lumber yards constantly. Linear feet is just length — a 10-foot 1×6 is 10 linear feet. Square footage adds width: that same 10-foot 1×6 is 10 × (5.5/12) = 4.58 square feet of surface area.

Why it matters: when staining or finishing, products list coverage in square feet, not board feet. A quart of Danish oil covering 150 sq ft won't cover 150 board feet of 8/4 stock — that stock has two faces, two edges, and if it's rough, a lot more surface area per board foot than you'd think.

11. Sandpaper Grits: CAMI vs. FEPA (P-grade)

Sandpaper comes in two grading systems and mixing them up wastes abrasive and ruins surfaces. CAMI (American) and FEPA/P-grade (European) differ more as grits get finer:

P80 ≈ 80 grit (same at coarse end)
P120 ≈ 100 grit
P150 ≈ 120 grit
P180 ≈ 150 grit
P220 ≈ 180 grit
P320 ≈ 220 grit

If you buy P220 paper thinking it's 220 grit, you'll skip a step in your sanding progression. Most sandpaper sold at big-box stores is CAMI; most European brands are FEPA. Check the back of the sheet — P-grade has the "P" prefix.

12. Moisture Content and Wood Movement

This one isn't a unit conversion exactly, but it's the most important measurement relationship in woodworking: for every 1% change in moisture content, most domestic hardwoods move approximately 1% of their width in the tangential direction (across the grain) and about 0.5% in the radial direction.

Practical translation: an 8-inch-wide flatsawn oak panel going from 12% MC (delivered from a damp supplier) to 7% MC (your heated shop in winter) will shrink roughly 8" × 5% × 1% per point = about 0.4 inches. That's almost half an inch. Plan your panel gaps, floating tenons, and tabletop buttons accordingly.

Wood movement isn't optional knowledge — it's what separates furniture that lasts from furniture that splits.

The Real Secret: Keep a Cheat Sheet in the Shop

I'm not suggesting you memorize all twelve of these perfectly. I've been doing this for years and I still look up obscure drill-bit decimals. What matters is knowing which conversions exist and roughly what they give you — enough to catch obvious errors before you make a cut.

Write the ones you use most on a piece of tape on the side of your tool cabinet. Mine has the board foot formula, the nominal-to-actual lumber chart, and the hardwood quarters system. Three years later it's still there, bleached and dusty. Best three minutes of label-making I ever did.