What this calculator does
A moth or butterfly spreading board has a central groove for the body and two side decks that support the wings while paper or glassine holds them in position. This calculator turns your specimen measurements into a board layout: groove gap, side deck width, full board width, paper strip cut width, and a pass-or-adjust check for a board you already own.
The key is that the calculator does not assume one “normal” moth. A tiny micro-moth, a bulky sphinx moth, and a broad butterfly all need different geometry. Measure the actual body width and intended spread wingspan, then adjust the clearance and margin to match how delicate the specimen is.
How to use it
- Choose the unit system you want to work in. Every label, input, and result will switch together.
- Measure the specimen body width at the widest part that must sit in the groove.
- Measure or estimate the final spread wingspan from wing tip to wing tip.
- Set your groove clearance per side. A zero value means the gap equals the measured body width; a larger value gives more handling room.
- Set the wing-tip margin. A common starting point is at least to beyond the wing tip so the tips are not on the board edge.
- If you are checking a board, enter its groove gap and one side deck width. The fit box will show whether the gap and side deck are large enough.
Formula explained
Groove gap = body width + (2 × chosen side clearance)
Side deck per side = (wingspan ÷ 2) + wing-tip margin − (groove gap ÷ 2)
Total board width = groove gap + (2 × side deck)
Paper strip width = half of the wing span outside the body + your chosen paper overhang, capped at the available side deck.
These are geometry calculations. They do not replace careful specimen handling, relaxing, pin placement, or taxon-specific mounting practice. For fragile specimens, use the calculator as a sizing guide and handle conservatively.
When this is useful
- Building a custom board: cut the groove and side panels to fit the specimens you actually collect instead of copying a one-size board.
- Buying an adjustable board: check whether the advertised gap and deck dimensions cover your target wingspan.
- Preparing a batch: compare a specimen against the board before relaxing and spreading, so you do not discover the board is too narrow mid-process.
- Cutting setting paper: get a strip width that covers the wing area without hanging over the board edge.
Common mistakes to avoid
Making the groove much wider than the body
A very wide groove can leave the body poorly supported and may make wing bases sit below the board surface. Add only the clearance you truly need for handling and pin access.
Ignoring the wing-tip margin
If the wing tip lands at the outer edge of the board, it is easier to bump while pinning paper, moving the board, or storing the setup to dry. The margin input keeps that protective space visible in the result.
Cutting paper wider than the side deck
Setting paper should cover the wing, but it should not hang over the edge of the board. The paper result is capped by the side deck to keep the cut practical.
FAQ
How wide should a moth spreading board gap be?
Base the gap on the actual body width of the moth, plus any side clearance you choose. A narrow groove supports the body and helps the wing bases sit near the deck surface. A groove that is too tight can rub scales or make it difficult to lower the pin cleanly, so the calculator lets you choose the clearance rather than forcing a fixed standard.
How do I know if my spreading board is wide enough?
Measure the full spread wingspan, then allow extra room beyond each wing tip. The calculator checks side deck width from the center groove outward, because that is the space the wings actually occupy. If the board fails the side-deck check, the wing tips are likely too close to the edge even if the specimen technically fits on the board.
What does the setting paper strip width mean?
It is a cutting guide for the paper or glassine strip used to hold a wing in place. The strip should cover the wing area, but the pins go through the paper into the board, not through the wing. The calculator estimates a strip that covers the wing plus your chosen extra coverage while staying within the side deck.
Can I use this for butterflies as well as moths?
Yes. The board geometry is the same for butterflies and moths: body in the groove, wings on the side decks, paper pinned over the wings. The handling may differ by specimen condition, wing shape, and fragility, so treat the result as a layout size and still use careful entomological technique.
Why does the side deck change when the groove gap changes?
The wing tip is measured from the centerline of the specimen, while the side deck begins at the edge of the groove. A wider groove moves the deck edge farther outward, so the remaining deck needed beyond that edge changes. The calculator uses that relationship to avoid overbuilding or underbuilding the side panels.