Pen Blank Calculator
Blank dimensions for pen kit mandrel and tube.
About this calculator
Pen turning starts with matching a wood blank to the metal tube it will be glued around, and this calculator works out the two dimensions that matter most: how big a square blank you need, and how long it needs to be. The minimum blank cross-section is whichever is larger of your desired finished pen diameter plus a 1/8" turning margin, or the tube's outer diameter plus a 1/4" wall allowance — the first constraint usually governs on slim, low-profile pens, while the second matters if you want a noticeably wider barrel than the tube alone would need. Blank length per barrel simply adds 1/16" of trim allowance to each end of the tube length, since you'll square the blank ends flush to the tube after gluing and need a little extra to true them up on the pen mill.
From there the calculator scales that per-barrel length by how many barrels your kit uses (most twist and rollerball pens use two) to get total material required, reports the drill bit size that matches your tube's outer diameter (in both mm and inches, since pen-kit tubes are sized metric but many shops still stock fractional bits), and computes how many barrels you can cut from a standard 5" pre-cut pen blank stick. It also flags minimum wall thickness — finished diameter minus tube OD, divided by two — as a sanity check: a very thin wall after turning is prone to blowing out under drilling or sanding pressure, so if that number comes out uncomfortably small, size up your finished diameter or choose a smaller tube kit.
Inputs
Results
Min blank cross-section (in)
0.63
Blank length per barrel (in)
2.23
How to Use This Calculator
- Enter Tube outer diameter (mm), Tube length (in), and Finished pen diameter (in).
- Set Number of barrels and Board thickness (in).
- Review Min blank cross-section (in) and Blank length per barrel (in).
- Use Total blank length (in) and Drill bit size (mm) to inform your decision.
How the result changes with Finished pen diameter (in)
| Finished pen diameter (in) | Min blank cross-section (in) | Blank length per barrel (in) |
|---|---|---|
| 0.25 | 0.53 | 2.23 |
| 0.38 | 0.53 | 2.23 |
| 0.75 | 0.88 | 2.23 |
| 1.25 | 1.38 | 2.23 |
What each input means
- Tube outer diameter (mm)
- Outer diameter of the brass pen tube. 7mm for slimline, 10mm for cigar/sierra kits.
- Tube length (in)
- Length of one brass tube barrel.
- Finished pen diameter (in)
- Desired maximum outer diameter of the finished pen barrel.
- Number of barrels
- Number of tube barrels in the kit (most pens have 2).
- Board thickness (in)
- Thickness of the lumber you plan to cut blanks from.
What each result means
- Min blank cross-section (in)
- Minimum square blank side dimension.
- Blank length per barrel (in)
- Required blank length for each barrel (tube + trim allowance).
- Total blank length (in)
- Total linear inches of blank material for all barrels.
- Drill bit size (mm)
- Drill bit diameter to match the tube OD.
- Drill bit size (in)
- Drill bit diameter in inches.
- Min wall thickness (in)
- Thinnest the wall will be at finished diameter. Below 1/8" risks breakage.
- Blanks per 5" stick
- How many barrels you can cut from a standard 5" pen blank.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersTube outer diameter (mm) = 7, Tube length (in) = 2.1, Finished pen diameter (in) = 0.5, Number of barrels = 2 = 5 input(s) provided
- Calculate Min blank cross-sectionMin blank cross-section = max(finishedPenDiameter + 0.125, tubeODInches + 0.25)0.625 = 0.625
- Calculate Blank length per barrelBlank length per barrel = tubeLengthIn + 0.1252.225 = 2.225
- Calculate Total blank lengthTotal blank length = blankLengthPerBarrel * numberOfBarrels4.45 = 4.45
- Calculate Drill bit size7 = 7
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator sometimes size the blank off the finished pen diameter and sometimes off the tube diameter?
It takes whichever produces the larger minimum blank side: finished pen diameter plus a 1/8" turning margin, or tube outer diameter plus a 1/4" wall allowance. On slim, low-profile pens the finished-diameter constraint usually governs since the tube is already close to the target size; on pens where you want a noticeably wider barrel than the tube needs, the tube-plus-wall constraint takes over instead.
How is the drill bit size determined, and why are both mm and inch values given?
Drill size is set equal to the tube's outer diameter directly, since you're drilling a hole just large enough for the brass tube to be glued in. Both units are shown because pen-kit tubes are manufactured to metric sizes, but many hobby shops and drill index sets in the US stock fractional-inch bits, so having both lets you match whichever set you own.
What does a very small minimum wall thickness number mean for my project?
Minimum wall thickness is finished pen diameter minus tube outer diameter, divided by two — the thinnest the wood will be once turned down around the tube. If that number comes out uncomfortably small, the wall is prone to blowing out during drilling or sanding; the fix is to increase your finished pen diameter or choose a kit with a smaller tube.
Why does "blanks per stick" sometimes seem low for a 5-inch stick?
Blanks per stick divides the standard 5" stick length by your blank length per barrel (tube length plus 1/8" total trim allowance), then rounds down, since a partial blank can't be used. Longer tube barrels or added trim allowance eat more of the 5" length, leaving room for fewer whole barrels — this ignores board thickness and cross-section, which is checked separately by the "Blanks From Thickness" flag.
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