1:25 and 1:50 are the two scales at which theatre groundplans, sections and models are most often drawn in the UK. A scale is a fixed ratio between the drawing and the real thing it stands for: at 1:25 the drawing is one twenty-fifth of full size, so a meter on stage becomes 40mm on the page, and at 1:50 it is one fiftieth, so a meter becomes 20mm. Choosing between them is really choosing how much of the stage you can fit on a sheet and how much detail survives once it is there.
The part that catches people out is that the larger number makes the smaller drawing. A set drawn at 1:50 is half the linear size of the same set at 1:25, and a quarter of the area, so 1:25 is the bigger, more detailed drawing and 1:50 the more compact overview. The two scales are exactly a factor of two apart, which turns out to be convenient in practice: it is why the same model, figure or doorway can be checked against either without much arithmetic, and why the two so often sit side by side in one designer's work.
Reading a scale drawing
The ratio is a promise that every length on the page holds the same relationship to the real thing, which is what lets a plan be measured rather than merely looked at. To go from the stage to the paper, take the measurement in meters and multiply by 40 at 1:25, or by 20 at 1:50. A 12-meter proscenium opening is therefore 480mm across at 1:25 and 240mm at 1:50, and a 200mm step is 8mm and 4mm.
Scale converter
4.2 m
168 mm drawn
4200 mm in the room draws 168 mm long at 1:25.
In practice nobody does that sum, because a scale rule does it for you. A scale rule carries several edges, each graduated in the real measurement for one ratio, so laid against a 1:25 plan its 1:25 edge reads distances straight off in meters with no conversion at all. Most rules used for this work carry both 1:25 and 1:50 among their scales, along with the others an architect or designer needs.
Because 1:25 and 1:50 are exactly a factor of two apart, reading a drawing on the wrong edge of the rule does not give an obvious nonsense but a plausible figure that is out by half or double: measure a 1:50 plan on the 1:25 edge and every dimension comes back at half its real size, and the error can run all the way to a wall built to the wrong measurement. Confirm a plan's stated scale before taking anything off the rule.
Every drawing states its scale in the title block and carries a scale bar, a short ruler printed at the drawing's own scale. The scale bar is worth trusting above the figure printed beside it in one situation: when a plan is printed or photocopied at the wrong size, the drawing and the bar stretch together, so a distance read against the bar stays correct even though the same distance read against a separate rule no longer is. Where an actual dimension is written on the plan, that written figure is the one to work from, because it is what the designer intended and the printed line may have moved.
Which scale, and when
1:25 is the everyday scale for detailed theatre drawing in the UK. Groundplans, sections and the model box are commonly built at it, because at 1:25 a doorway, a tread, a molding or the swing of a flat is large enough to draw clearly and to measure with confidence. A reader can take real construction information off a 1:25 plan.
1:50 comes in when 1:25 becomes unwieldy. A large stage, an opera set, or a drawing that has to take in the whole building can run to a sheet too big to print or handle at 1:25, so it is dropped to 1:50 to bring it onto a manageable size, accepting coarser detail in return. The same production will often use both: a working groundplan at 1:25 for the set itself, and a wider drawing at 1:50 where the point is to show the whole stage, the wings, the crossover and the way the set sits in the venue. Neither is the correct scale in the abstract; each answers a different question, and the choice usually follows the size of what you are drawing and how much of it has to fit on one sheet.
| Scale | Typical use | A 12 m proscenium draws at |
|---|---|---|
| 1:25 | Design groundplans, workshop drawings, lighting plots | 480 millimeters |
| 1:50 | Full stage sections, small venues, planning overviews | 240 millimeters |
| 1:100 | Site plans, large concert houses, fit plans for tours | 120 millimeters |
| 1:24 (1/2" = 1'-0") | North American design groundplans | 500 millimeters |
| 1:48 (1/4" = 1'-0") | North American full-stage views | 250 millimeters |
Behind the table is a single rule: draw at the largest scale that still keeps the whole subject on one sheet. Detail costs paper, and a groundplan that runs onto a second sheet stops being one object you can reason about. A studio 10 meters deep sits comfortably at 1:25; an opera house stage 30 meters deep needs 1:50 to make the same sheet; a festival stage in an arena reaches 1:100 before it reaches a bigger printer.
Paper sizes, and what fits where
The A series doubles in area at every step: A4 is 210 by 297 millimeters, A3 is 297 by 420, A2 is 420 by 594, A1 is 594 by 841 and A0 is 841 by 1,189. That regularity is what makes the arithmetic worth doing once. Working at 1:50, the long edge of an A1 sheet stands for 841 x 50, or about 42 meters of stage, which is why A1 at 1:50 is the classic full-stage format: the whole room, the wings and the title block beside them.
At 1:25 the same A1 sheet covers 594 x 25 across its width, about 14.8 meters, which takes a modest stage and its wings and not much more. That is the trade the table sets out in another form: twice the detail, half the room. Designers on larger shows get both by drawing at 1:25 and printing at 1:50 for the overview, which is legitimate only because the two prints come from one drawing whose scale never changes.
The model and the scale figure
The scale a plan is drawn to is usually the scale its model is built to, so that plan and model can be read against one another. In the UK the white card model and the finished model are both commonly made at 1:25. At that scale a model of an ordinary stage is large enough to hold real detail and to reach a hand into, and small enough to carry to a production meeting; very large stages are sometimes modeled at 1:50 instead, for the same reason their plans are.
A model built to scale usually has a scale figure standing in it, a small person made to the same 1:25, because a measurement means most when there is a human body beside it. A 1.8-meter actor is 72mm tall at 1:25 and 36mm at 1:50, and standing that figure on the model is the quickest way to see that a doorway is too low, that a rostrum has quietly become a climb, or that a handrail sits at the wrong height. The figure is also where scale turns into a sightline: lower your eye to its head and you begin to see what someone in that seat would see, which a flat plan on its own cannot tell you.
The same scales in other conventions
1:25 and 1:50 are UK habits, tied to the metric system and to A-series paper. Elsewhere the drawing looks a little different. In the United States, where drafting is imperial, the equivalent everyday scales are a half-inch and a quarter-inch to the foot. A half-inch to the foot works out at 1:24 and a quarter-inch at 1:48, close enough to 1:25 and 1:50 that the two traditions line up neatly without being identical, and a designer crossing between them treats a US half-inch model much as a UK 1:25 one.
The near-equivalence is useful to know precisely so that you do not assume it. A plan from another country states its own scale, and that statement, rather than the ratio you expected, is what to read it by. The same caution applies to any drawing that has passed through a photocopier or a foreign print shop.
Scale in Setplot
In a drawing made by hand or in CAD, scale is something you draw to: you fix on 1:25 or 1:50 at the outset and set down every line at that ratio. In Setplot the plan is metric and to scale by construction, because scenery is held as physical objects with real dimensions measured from the plaster line and the centre line, not as shapes drawn at a chosen size. The measurements are the drawing, so scale becomes something you choose when you view or print rather than something you commit to at the start: the same design can be output at 1:25 for detailed work or 1:50 for an overview without being redrawn, and the arithmetic a scale rule saves you is not needed in the first place.
Because the measurements are real throughout, the rest follows from them. The white-card 3D view is generated from the same figures, so it arrives already at the scale of the plan it came from and puts back the heights a plan leaves flat; audience sightlines can be checked from particular seats; and a design can be tested against a second venue's dimensions for a tour or a transfer without starting the drawing again. The core drawing tools are free to use, including a public board before you make an account, which makes it a reasonable place to see how a plan at scale behaves.
None of this replaces a model cut at 1:25 by hand, or a scale rule and a sharp pencil. A set you can pick up and turn under a lamp is its own kind of knowledge, and reading a scale rule is a skill worth having in a rehearsal room where the drawings are on paper. What the digital plan removes is the redrawing and the wrong-edge error, while keeping every view measured from the same real dimensions.
Whichever ratio a drawing carries, the number in the title block is a contract. It fixes the exact size of everything on the page, so that a set thought out in a model box a few centimeters high can be built at full size on a stage and come out as the designer meant it.