A measured outline · one free calculator
Two foci.
One smooth ellipse.
Enter the full width and height of your ellipse. Find its focus positions and the working string segment for two ends fixed at the foci.
Your ellipse
Full diameters, not distances from the centre. Major axis runs horizontally in the layout.
Plain decimals only, up to 12 fractional places. Positive axes up to 1,000,000 mm equivalent; minor / major at least 1 / 1,000,000. These are software limits.
Layout reference
Cut arithmetic with that total:
Equal-scale reference drawing. Dots mark focus centres, not pin diameters. Very slender shapes may appear as a line at screen scale.
Coordinates ·
Origin at centre; x along major axis. Fractions are exact. Focus positions use an approximate square root.
| Point | x | y | Precision |
|---|
SVG native dimensions are in millimetres, with 3% major-axis padding per side. Printing or cutter import may rescale it: verify a known dimension independently.
1. Divide the diameters
A 1000 × 600 mm outline has semi-axes 500 and 300 mm. This separates the overall size from centre-based measurements.
2. Locate the two foci
The example focus offset is 400 mm from centre, giving 800 mm between focus centres. The calculated offset is a point location, not a hole size.
3. Check the working segment
The taut focus-to-pencil distances add to 1000 mm. The entered 40 mm total extra gives 1040 mm cut arithmetic; knot loss is not predicted.
Before you mark
Use a flat layout and check the centre and perpendicular axes. The model assumes point pins and pencil, unstretched string and ideal geometry. Pin diameter, pencil offset, stretch and fabrication tolerances are outside it.
Read the complete marking guide and worked examples →
What happens when the axes are equal?
The ellipse becomes a circle and both foci are at the centre. Use a centre-and-radius construction; there is no separate two-pin spacing.
Does changing units alter the shape?
Each field keeps its exact physical value through unit changes. Displayed conversions may round to 12 places. Editing an individual field replaces that field's retained value. The inch conversion is exactly 25.4 mm.
Where do my measurements go?
The calculator and exports run in your browser. No measurements are sent by the application, and it does not save them between visits. Hosting still receives ordinary page requests. See privacy details.