When to use this calculator
Find an ellipse’s semi-major axis from periapsis and apoapsis distances. For an ideal elliptical orbit; use centre distance instead of altitude.
Explore the orbits & escape calculators for other questions in this topic. For a related calculation, use the Orbital Period From Mu Calculator: Find orbital period from central gravitational parameter and circular-orbit radius.
PUTTING IT INTO PRACTICE
Orbit Semi Major Axis: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Apoapsis centre distance | 12 |
| Periapsis centre distance | 8 |
| Result | 10 input distance units |
- Check the input labels and enter the example values above.
- Apply the relationship: Semi-major axis = (apoapsis+periapsis)/2.
- The result is 10 input distance units, rounded for display.
How to interpret your result
For an ideal elliptical orbit; use centre distance instead of altitude.
Use radius from the centre of the attracting body, not altitude above its surface. Choose the gravitational parameter of the correct primary. Drag, rotation and other bodies are excluded from these simple models.
Frequently asked questions
Can I use altitude directly as orbital radius?
No. Add the central body’s radius to altitude when a formula requires distance from its centre.
When is the orbit semi major axis calculator useful?
Find an ellipse’s semi-major axis from periapsis and apoapsis distances. For an ideal elliptical orbit; use centre distance instead of altitude.
How do I use the orbit semi major axis calculator?
Enter apoapsis centre distance, periapsis centre distance. The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Semi-major axis = (apoapsis+periapsis)/2.
What should I check before using the result?
For an ideal elliptical orbit; use centre distance instead of altitude.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see NASA: orbits and Kepler’s laws ↗. This is a topic reference, not an external review or endorsement of this tool.
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