When a blanket concept is applied to the actual reactor design, various structural changes such as material thickness, material volume fractions, etc., are made to adjust the overall design to meet lifetime and material stress requirements. After these changes, the new tritium breeding ratio (TBR) is required to be easily and quickly estimated. Hence, an analytical TBR formula was derived by separating absorption, scattering, neutron multiplication, and tritium production cross sections at high energies above the multiplication reaction threshold from those at low energies near the thermal energy. The formula was applied to three blanket types. The TBR values calculated by this formula agreed with those of an ANISN transport code within 5%.