Water homeostasis generally occurs without conscious effort; however, estimating requirements can be necessary in settings such as health care. This review investigates the derivation of equations for estimating water requirements. Published literature was reviewed for water estimation equations and original papers sought. Equation origins were difficult to ascertain and original references were often not cited. One equation (% of body weight) was based on just two human subjects and another equation (ml water/kcal) was reported for mammals and not specifically for humans. Other findings include that some equations: for children were subsequently applied to adults; had undergone modifications without explicit explanation; had adjusted for the water from metabolism or food; and had undergone conversion to simplify application. The primary sources for equations are rarely mentioned or, when located, lack details conventionally considered important. The sources of water requirement equations are rarely made explicit and historical studies do not satisfy more rigorous modern scientific method. Equations are often applied without appreciating their derivation, or adjusting for the water from food or metabolism as acknowledged by original authors. Water requirement equations should be used as a guide only while employing additional means (such as monitoring short-term weight changes, physical or biochemical parameters and urine output volumes) to ensure the adequacy of water provision in clinical or health-care settings.