Guides

Scale 1:50 at A1 printed on A3: why it becomes 1:100

A drawing at scale 1:50 on A1, printed on A3, is at 1:100. A3 is A1 halved twice. Each halving shrinks the sides by 1/√2, and two of those steps together make exactly a half, so every length on the A3 print is half as long. That's why a sheet should say 1:50 @ A1 and carry a scale bar, which stays true however small the print.

The drawing above shows it: a ground floor and site plan at 1:50 on an A1 sheet, and the same sheet printed on A3. On the A3 every length is half as long, so the plan measures 1:100 even though its title block still says 1:50. The scale bar shrank with the drawing, so it still reads true.

What a scale of 1 to 50 says

1:50 means one unit on the paper stands for fifty in the building. A wall 50 units long is drawn 1 unit long. It's one of the reductions ISO 5455 permits: 1:2, 1:5, 1:10 and their decimal multiples, so 1:20, 1:50, 1:100, 1:200, 1:500 and on up to 1:10000. Nothing in between is a standard scale.

By convention, floor plans and elevations go at 1:100 or 1:50, sections at 1:50 or 1:20, details at 1:10 to 1:1, and site plans at 1:500 to 1:1250. You pick the scale so the detail reads, then put the drawing on the smallest sheet it fits at that scale with room for its dimensions and notes.

The catch is that the ratio is only true on the paper it was drawn for. Print the sheet on anything else and the ratio changes, but the words in the title block don't.

Why two A sizes down halves the scale

The A sizes come from ISO 216, and each one is the size above cut in half across its long side. The trimmed sizes in millimetres:

Halving the area while keeping the same shape means each side shrinks by 1/√2, about 0.71. Do that once and the sides are 0.71 of what they were. Do it twice and you get 1/√2 × 1/√2, which is 1/2. You can check it on the table: the A3 short side, 297, is half the A1 short side, 594. The A3 long side, 420, is half the A1 long side, 841, give or take the rounding to whole millimetres.

So an A1 sheet printed on A3 has every line half as long. A wall drawn at 1:50 now measures as if it were drawn at 1:100. Because every A size has the same proportions, the whole sheet fits the smaller paper exactly, with nothing cropped and nothing stretched.

One size down is the awkward one

Print the same A1 sheet on A2 and the lengths shrink by 0.71, once. The drawing is no longer at 1:50, and it isn't at 1:100 either. It sits somewhere between the two, at a ratio that isn't one of the permitted scales and that no scale rule has a side for.

That's why offices reduce by two sizes when they reduce at all. A1 to A3 and A0 to A2 each land on a clean half, and a half of a permitted scale is usually another permitted scale. One step gives you a print you can read but can't measure.

A drawing scale converter, A1 to A3

Two sizes down doubles the second number of the ratio. Here are the common ones:

The rule is the same whichever pair you start from: two A sizes down, the scale halves. Four sizes down, A0 to A4, it halves twice.

Write 1:50 @ A1, not just 1:50

ISO 5455 has the scale written in the title block as SCALE 1:50. Where a sheet holds views at more than one scale, the main scale goes in the title block and every other view is titled with its own.

The trouble is that sheets travel. An A1 set gets reduced to A3 for a meeting, a client prints it on A4, a planning portal shrinks it to fit a screen. The title block still says 1:50, and someone with a scale rule gets every length wrong by a factor of two.

Writing 1:50 @ A1 fixes that. Anyone holding an A3 print can see the paper it was drawn for and work out that their copy is at 1:100. It costs a few characters in the scale field. If you're setting up the title block itself, the ISO 7200 title block guide goes through each field on every sheet size.

Add a scale bar, because it shrinks with the print

A written ratio is a claim about the paper. A scale bar is part of the drawing. It's drawn in the drawing's units, so when the sheet is reduced the bar is reduced by exactly the same amount as the walls beside it. Its marked lengths still match the plan, on A1, on A3, or on a screen at any zoom.

That's why the scale bar in the drawing above still reads true on the A3 print while the title block doesn't. Put one on every plan, section and elevation, near the view title, and a reader never has to trust a ratio that might not apply to their copy.

A scale bar is for getting a sense of size, not for setting out. ISO 129-1 has the dimensions give the true figure whatever the scale, and ISO 5455 says a drawing is never scaled off with a rule. If a length matters, it should be dimensioned. The bar is there for the person checking whether a room looks right, not for the person building it.

Lettering and the reduced print

Everything halves on an A3 reduction, including the text. Lettering that's comfortable on A1 may be hard to read once it's half the height. If you know a set will mostly be read as A3 reductions, size the lettering so it still reads at half height. The text height for scale guide covers the sum between printed height and drawing height.

How isogram keeps the scale on the paper

In isogram you draw at full size in Draft, then lay out sheets with viewports in Sheets. A sheet knows its paper size and each view knows its scale. When you publish the set as one PDF, each sheet is printed at its own paper size, so the scale on the sheet is the scale on the paper. A 1:50 view on an A1 sheet comes out of the PDF at 1:50 on A1, not shrunk to fit something else.

Text height is set as the height on the printed sheet, in millimetres, and the app works out its size on the drawing from the view's scale. Change a view from 1:50 to 1:100 and the lettering stays the size it prints at. Before a set is issued, isogram reads every sheet for the things a senior drafter would catch, including lettering too small to print, an empty title block field, or two sheets with one number.

If you want to reduce to A3 for a meeting, you still can. Just write the paper size next to the scale and keep the scale bar on the sheet, and the A3 copy tells its own story. isogram's public beta opens in December 2026 on the Mac and Windows, with Linux following in an update, and it's free to test. You can join the list to get the download link.

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2 November 2026