How it fits

Designing a watch on a screen is easy and ordering six parts that go together is not. This is the short version of what actually decides it, so that when the designer refuses a combination you know what it is talking about.

A case is cut for a footprint, not a calibre

This is the idea everything else hangs off. Nobody machines a case for the NH35 specifically — they machine it for a 27.4 mm disc, 5.32 mm tall, with the stem at a certain height and a dial 28.5 mm across. Every movement built to those numbers drops in.

So an NH35 (date), an NH36 (day-date), an NH38 (no date), an NH34 (GMT), an NH70 (skeleton) and a donor 7S26 are all the same fitment problem. Change your mind about a date window and you do not change the case. That is why the Seiko modding world is as big as it is: one footprint, one dial size, one hand size, and thousands of parts made to them.

Choose the footprint first. It decides the case, the dial, the hands and most of the budget. The individual calibre inside it is nearly a detail.

The six numbers

Everything the designer checks comes down to these. Five of them are the reason parts arrive and do not go together.

Number	What it decides	What goes wrong
Movement diameter	Whether the movement sits in the seat	Too big and it will not go in at all. Too small and it needs a movement ring; a poor one lets the movement turn in the case and the hands drift off the markers.
Movement height	Whether the caseback closes	The commonest failure. A 7750 is 7.9 mm tall and most 38 mm dress cases are about 4 mm deep. No amount of wanting changes it.
Dial diameter	Whether the dial clears the opening	A dial too large fouls the chapter ring; too small and there is a visible crescent of nothing at the edge.
Hand hole sizes	Whether the hands stay on the pinions	Three numbers — hour, minute, second. Seiko is 1.50/0.90/0.20 and Miyota is 1.52/1.00/0.17, which looks like nothing and means Seiko hands are loose on a Miyota and Miyota hands will not press onto a Seiko.
Lug width	Whether the strap goes between the lugs	Simple, and still the most-returned part. Measure it; do not trust a listing.
End-link fitment	Whether a bracelet sits flush	A curved solid end link is machined to one case. An SKX end link will not sit on an SRPD even though both are 22 mm.

The layout is the movement's, not yours

A dial has to be drawn around what the movement does. This is not a rule the app imposes; it is where the wheels are.

The date window goes where the movement's aperture is. Three o'clock on most, four-thirty on a VK63, six on a few. You do not choose it — you choose a movement that puts it where you want it.
And it goes as far out as the date ring reaches, no further. The hour is only half of it. What shows through the window is a printed ring turning underneath, and that ring is a real part: a Miyota 9015 carries one 23.40 mm across, so its numerals never come past 11.7 mm from the centre. Cut the aperture at 12.7 and the watch comes back with a window onto bare movement. A drawing will not warn you, because a drawing paints the numeral in wherever the hole is — so the dial studio checks it instead.
A chronograph puts its registers where its wheels are. A 7750 is 6–9–12 and cannot be anything else; a Sea-Gull ST19 is two registers at 3 and 9; a Ronda 5030.D is three at 3, 6 and 9. Choosing the movement chooses the symmetry of the dial.
A thirty-minute register is graduated in thirty minutes. Printing a sixty-minute scale on it makes the watch read wrong by a factor of two, and it is the mistake most visible on cheap chronograph dials.
A day-date movement needs a wider cut-out than a date movement, so an NH35 dial will not do for an NH36 even though the movements are otherwise identical.
A skeleton or open-heart movement needs the matching aperture. The movement decides the dial before you have drawn anything.

What a complication asks of the build

Every complication is something the movement does, so each one arrives with conditions you can't design away. Some are about the dial, some the case, and some are about how the watch has to be treated once it's built. The Movements page can list every movement with a given complication.

Moon phase. The movement decides where the moon is: twelve or six on most, between one and two on a Sea-Gull ST2153, and inside the chronograph's hour counter on a Valjoux 7751. The dial is cut for that aperture and nothing else. A moon module also adds height; the SW288-1 is a millimetre taller than the SW200 it is built on, which is why several cases here had to be cut deeper for it.
Calendars have a quiet hour. The calendar works engage over several hours around midnight, and forcing a correction then can damage the wheels. Ernst Benz's instructions for a 7751 say not to set the day and date between 8 pm and 2 am, or the moon between 8 pm and 6 am. Set the time past that window first, then correct the calendar.
Perpetual calendar. It knows every month's length and the leap years, so it needs correcting by hand only when the century rule skips a leap year, next on 28 February 2100. What it can't forgive is being left to stop: on many, a perpetual that runs down has to be brought forward through its correctors one indication at a time, which is slow and easy to get wrong. The ones on the reference shelf are made for brands, not sold loose.
Pointer date. The date is read by a long central hand against 1 to 31 printed round the dial, so the dial carries a second scale and the minute track has to make room for it.
GMT. A 24-hour hand is only half of it; the other half is which hand you can set on its own. On a caller GMT such as the Sellita SW330-2, the quick-set moves the 24-hour hand, so the hour hand keeps home time. On a flyer, such as Tudor's MT5652, the local hour hand jumps instead, which is what a traveller wants. Pair either with a 24-hour bezel and it reads a third zone.
Chronograph. The pushers need bores in the case at the movement's own heights and positions. A 7750-footprint chronograph is 7.9 mm tall, so the watch will be thick whatever the dial does. A column wheel (the La Joux-Perret L100, the Sea-Gull ST19) gives a crisper, more even push than a cam, and that difference is the reason to pay for one.
Power reserve. The gauge goes where the movement drives it, between nine and eleven on the ST2153. It is also the one complication that tells you how the watch is being worn.
Tourbillon. The cage is seen through an aperture at the movement's hour, six on the Sea-Gull ST8000, and the case wants a display back, because half the point is the other side. A hand-wound tourbillon is wound every day or two; the ST8000 runs about 42 hours.
Discontinued movements. The Ronda 706.B and the Miyota 6P80 are both marked discontinued by their makers. They still fit everything they fitted. Buy a spare with the first one if the watch is meant to be serviced rather than replaced.

A complication is a promise the movement makes. The dial only has to keep out of its way, and the owner only has to know its rules.

Bezels are instruments

A bezel scale is read against a hand. Put a scale on a watch that has no hand to read it with and you have made an ornament:

A tachymeter is read against a central chronograph seconds hand. Without one it measures nothing.
A 24-hour bezel is read against a fourth hand. On an NH35 it is decoration; on an NH34 or a Miyota 9075 it is a second time zone.
A dive bezel is read against the minute hand and is the one scale that works on any three-hand watch — which is why almost every diver has one.

What thin costs

Thinness is bought from the movement, not from the case. Working down the catalogue: a Valjoux 7750 is 7.9 mm, an NH35 is 5.32, a Miyota 8215 is 5.67, an ETA 2824 is 4.60, a Miyota 9015 is 3.90, an ETA 2892 or Sellita SW300 is 3.60, and a Ronda 715 is 2.50.

Add roughly five millimetres for a crystal and a caseback and you have the thinnest the finished watch can be. A watch under 8 mm thick is a quartz watch, and there is no argument to be had about it. If a design needs to be thin and mechanical, the 2892 family is the answer and it costs three times a 2824.

Lug-to-lug, not diameter

Diameter is the number in the listing and lug-to-lug is the number that decides whether a watch fits a wrist. A 44.3 mm Turtle has a 47.7 mm lug-to-lug and wears smaller than plenty of 42 mm watches, because a cushion case has no lugs sticking out of it. Sort the case table by lug-to-lug rather than diameter and the list rearranges itself completely.

As a rough guide, lug-to-lug should not exceed the flat width of your wrist. Under about 47 mm suits most wrists; over 50 mm needs a large one.

The order to buy in

Buying in the wrong order is how a build ends up with a drawer of parts that nearly fit.

Movement. It sets the footprint, and therefore everything else.
Case. Cut for that footprint. Check the seat diameter and the depth against the movement, not against the style name.
Dial. Sized to the footprint, with the right calendar cut-out and the right apertures.
Hands. Matched to the pinion sizes. Buy two sets — the first set is where people learn that fitting hands is a skill.
Crystal and bezel insert if the case did not come with them.
Bracelet or strap. Last, and the easiest thing to change afterwards, which is why it should never hold up the rest.

And buy the tools before the parts: a movement holder, a hand-setting press, a case-back wrench and a dust blower. A watch assembled with tweezers and optimism has fingerprints on the dial.

What this app does not know

Being clear about the edges is part of being useful.

Stem length and crown height. A movement in a case it was not sold with usually needs the stem cut to length, and the app has no data for that. It is a five-minute job and it is not optional.
Crystal height. Hand clearance under a flat crystal is only estimated, from the case depth. A tall hand stack — an NH34, a thick applied dial — can foul glass the numbers said was fine.
Where the date numerals actually print. Makers publish a movement's diameter and almost never its date ring's. Only the Miyota 9-series carries a measured one here, off Miyota's own drawing; every other ring is worked out from the movement's diameter using that same margin. So the dial studio says plainly which it is using, and a worked-out ring stays quiet unless the window is clearly off it. Measure the disc before you commit a dial to a printer.
Tolerances between sellers. Two vendors' versions of the same case style differ by a few tenths of a millimetre. Measure before you order a crystal or a bezel insert.
Prices. Indicative street prices for single units, and they move. They are here to show the shape of a budget, not to be quoted at a supplier.
Water resistance. The figure on a case is the figure for that case with its own gaskets, assembled properly and pressure tested. A case opened on a kitchen table is not rated for anything until somebody tests it.

Signing it

A maker's mark is the one part of the design that is entirely yours, and there are exactly two places to put it, at wildly different prices.

Engraved on the caseback. A laser or rotary engraver, one setup, and it costs about the same as a strap. Almost every small-run watch is signed here and nowhere else.
Applied on the dial. Cut from brass, polished, footed and glued. It needs tooling, and the tooling is usually the most expensive single line on a bespoke dial — several hundred pounds before you have one dial to show for it.
Printed on the dial sits between the two: one extra pass at the dial printer, and it looks printed, because it is.

Whichever you choose, one number decides whether it can be made. A dial logo is about three millimetres wide, and the finest line in the design has to survive at that size: 0.16 mm and over is comfortable, 0.10 to 0.16 mm is a good printer's limit, and under 0.10 mm fills in when printed and cannot be cut at all. The logo studio reports that number as you draw, next to a preview at actual size — which is the only honest way to judge a three-millimetre mark on a screen.

Design the mark at the size it will be used, not at the size it is comfortable to draw. Almost every logo that looks good at three hundred pixels is a smudge at three millimetres.

Where the reference calibres come from

The hundred and one generated reference movements — the Rolex 3135, the Lange L043.1, the Omega 321, the 1969 El Primero — are carried over from Calibre — the movement site in the folder next to this one, where each one is drawn part by part. You cannot buy any of them as a loose ébauche, which is why the designer only offers them in Concept mode: fitment worked out from dimensions rather than from a supplier's list, clearly labelled as such.

Concept mode is not a fantasy setting. The dimensions are real and the arithmetic is the same. It just stops pretending that anyone will sell you the movement.

One more reference calibre is written by hand rather than generated: the Vaucher VMF 5401/50, a micro-rotor perpetual calendar with a moon, 4.2 mm thin. Vaucher supply it to partner brands, so it is a Concept movement too. Unlike the generated entries it carries a layout, so its day, date, month and leap-year counters are drawn.