A rollout as previously described is the best way IMO. I use a blob of paint from a paint pen on my tire and simply ride it down my concrete driveway. Then I measure the distance between the marks on the concrete. Then if you measure in inches, then convert to millimeters. Google is your friend here if you are mathematically challenged.... inches to mm

If you want to calculate it, then it'll only be a rough estimate as two different model tires of the same printed size are unlikely to be the same. Especially if one is knobby tread and the other little to no tread. But find the ISO or ETRTO size of the tire. Usually printed or embossed in the tiny print on the sidewall. A 20 x 1.5" tire is a rare tire according to Sheldon Brown's tire sizing page and since this is a recumbent, it might a 406 BSD tire and not the same BSD of other 20" tires.

Once you know the ISO size the know that tire width is also about the same as the tire height from the bead seat. The ISO size is in mm. Don't use the inch sizes at all, they lie. The larger number is your tires BSD in mm and the smaller the tire width in mm.

So the formula for your circumference is:

((2 x tire width) + BSD) x 3.14 = your correction factor.