How this calculator works
The segment halfway between the bases has the arithmetic mean of their lengths. The step table expresses that mean as half of each base added together. Reversing the equation gives either missing base. No height or leg measurement is needed. Equal bases are accepted as the parallelogram limit, where the midsegment has the same length as each base.
Formula
m = (a+b)/2; a = 2m-b; b = 2m-a.
Worked example
For bases 8 m and 4 m, the half-base contributions are 4 m and 2 m. Their sum gives a 6 m midsegment. If the midsegment is 9 m and base b is 6 m, base a is 18-6 = 12 m.
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Frequently asked questions
Is the midsegment a diagonal?
No. A diagonal joins opposite vertices. The midsegment joins the midpoints of the non-parallel sides.
Does slanting the legs change this length?
Keeping both base lengths fixed keeps the midsegment length fixed, even when the height or horizontal offset changes.
What limits a missing base?
The sum of the bases is twice the midsegment. The known base must be smaller than that sum for the other base to stay positive.