An A10 page (landscape) is 37 x 26 mm. The sheet is A3, 297 x 420 mm, a built-in preset in FitPrint. The layout below is what FitPrint produces with its defaults: Fit most packing, Original size scaling, a 5 mm gap between items and a 5 mm side margin. Nothing is scaled: each item is placed at 37 x 26 mm, its true size.

Where each item lands

Positions are millimetres on the A3 sheet as FitPrint wrote them, with y measured from the bottom edge.

x (mm)	y from bottom (mm)	Width (mm)	Height (mm)
10.71	388.14	37	26
58.43	388.14	37	26
106.14	388.14	37	26
153.86	388.14	37	26
201.57	388.14	37	26
249.29	388.14	37	26
10.71	356.29	37	26
58.43	356.29	37	26
106.14	356.29	37	26
153.86	356.29	37	26
201.57	356.29	37	26
249.29	356.29	37	26
10.71	324.43	37	26
58.43	324.43	37	26
106.14	324.43	37	26
153.86	324.43	37	26
201.57	324.43	37	26
249.29	324.43	37	26
10.71	292.57	37	26
58.43	292.57	37	26
106.14	292.57	37	26
153.86	292.57	37	26
201.57	292.57	37	26
249.29	292.57	37	26
10.71	260.71	37	26
58.43	260.71	37	26
106.14	260.71	37	26
153.86	260.71	37	26
201.57	260.71	37	26
249.29	260.71	37	26
10.71	228.86	37	26
58.43	228.86	37	26
106.14	228.86	37	26
153.86	228.86	37	26
201.57	228.86	37	26
249.29	228.86	37	26
10.71	197	37	26
58.43	197	37	26
106.14	197	37	26
153.86	197	37	26
201.57	197	37	26
249.29	197	37	26
10.71	165.14	37	26
58.43	165.14	37	26
106.14	165.14	37	26
153.86	165.14	37	26
201.57	165.14	37	26
249.29	165.14	37	26
10.71	133.29	37	26
58.43	133.29	37	26
106.14	133.29	37	26
153.86	133.29	37	26
201.57	133.29	37	26
249.29	133.29	37	26
10.71	101.43	37	26
58.43	101.43	37	26
106.14	101.43	37	26
153.86	101.43	37	26
201.57	101.43	37	26
249.29	101.43	37	26
10.71	69.57	37	26
58.43	69.57	37	26
106.14	69.57	37	26
153.86	69.57	37	26
201.57	69.57	37	26
249.29	69.57	37	26
10.71	37.71	37	26
58.43	37.71	37	26
106.14	37.71	37	26
153.86	37.71	37	26
201.57	37.71	37	26
249.29	37.71	37	26
10.71	5.86	37	26
58.43	5.86	37	26
106.14	5.86	37	26
153.86	5.86	37	26
201.57	5.86	37	26
249.29	5.86	37	26

What other settings place

Settings	Items per sheet	Placed size
Fit most, 5 mm gap and 5 mm margin (default)	78	37 x 26 mm
Stack, 5 mm gap and 5 mm margin	13	37 x 26 mm
Fit most, no gap or margin	128	37 x 26 mm
Fit most, 10 mm gap and 10 mm margin	66	37 x 26 mm
Stack, no gap or margin	16	37 x 26 mm

Turned portrait, 121 fit: A10 pages on A3 (fit most, no gap or margin).

What the status line says

With 78 A10 pages (landscape) in the list, FitPrint's status line reads exactly "78 items · 1 A3 sheet". Add one more and it reads "79 items · 2 A3 sheets": the extra item starts a new sheet rather than shrinking anything.

How to lay this out in FitPrint

Drag your files in. Open FitPrint and drag your A10 page (landscape) files onto the window. Each file appears in the list with its size in the caption.
Choose the A3 sheet. Click A3 in the sheet preset grid. FitPrint then lays the items out on a 297 x 420 mm sheet.
Keep scaling on Original size. Original size places every item at 100 percent, so a printed item measures exactly what the file says it measures.
Leave packing on Fit most. Fit most packs as many items per sheet as fit. With the default 5 mm gap and 5 mm side margin that is 78 A10 pages (landscape) per A3 sheet.
Change the gap or side margin only if you need to. The defaults are a 5 mm gap and a 5 mm side margin. The settings table on this page shows what every other recorded setting places on this sheet.
Export and print at actual size. Click Export PDF, then set Scale to 100 percent and Pages per Sheet to 1 in the print dialog so nothing re-scales the exported file.

78 A10 pages (landscape) fit on one A3 sheet at actual size

Where each item lands

What other settings place

What the status line says

How to lay this out in FitPrint

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