This will get you really close if a rollout of the wheel measurement seems inconvenient.
Measure from the center of the axle to the outer radius of the tire. Where the rubber hits the road. Maybe reduce that number by a couple of mm to account for tire squish while riding.
Multiply that measurement, times Pie, times 2.
So, here's an example.
(Radius)x(2)x(Pie)
342x2x3.147=2155
Radius=342
Pie=3.147