According to the cern website it is actually 50km but on a much smaller pipe. The total volume 15000 cubic meter. For a 11 feet wide tunnel it would be about 1.7 km long.
So the tunnel the LHC is in has a circumference of 26.7 km. But the LHC has two parallel beam lines running in opposite directions. I assume that’s where the discrepancy between our numbers came from. But both beam lines are in the same vacuum vessel.
