BOD and COD number in faecal sludge waste from the septic tank is highly variables. Duri Kosambi Faecal Sludge Treatment Plant (IPLT Duri Kosambi) as one of the faecal sludge treatment of DKI Jakarta Province has Sludge Acceptance Plant and Sludge Basin system with influent values of BOD and COD sometimes exceed the range of ratio BOD/COD theoretical. However, the range of the ratio in septic sludge show 0.3-0.4 except for some occurrence. Laboratory chemical testing could estimate the dilution factor correctly of BOD five days values using the result of the COD test. This study aims to analyze the dilution factors of the five days of biochemical oxygen demand (BOD5) from BOD and COD values comparison. It takes some value of BOD/COD ratio actual range, and dilution factors obtained from calculating that range times COD values divided by percentage of oxygen consumption in five days (ΔDO) from the measurement of dissolved oxygen in the laboratory. When we could determine the dilution factors in BOD test based on COD values, so that no errors occured of the BOD5 number because there is enough oxygen to be consumed by microorganisms to oxidize almost all dissolved organic and some organic substances suspended on the fifth day.