A cell design is presented for experiments with hydrostatic pressures up to 25 kbar. The pressure‐transmitting medium around the sample is candle wax, which is sealed in the cavity with molybdenum disulfide impregnated nylon and aluminum disks. This approach showed no detectable leakage to 25 kbar. Internal cavity pressure is computed from measurements of the applied load on the top piston, the transmitted load to the lower piston, and the frictional forces transmitted by the seals into the pressure vessel and the cell body. Internal pressure thus measured and phase‐transition pressure data from RbCl and KBr are all within 9% of each other for the pressure range 3–25 kbar.