The term human equivalent is used in a number of different contexts. This term can refer to human equivalents of various comparisons of animate and inanimate things.

Animal models are used to learn more about a disease, its diagnosis and its treatment, with animal models predicting human toxicity in up to 71% of cases.^[1] The human equivalent dose (HED) or human equivalent concentration (HEC) is the quantity of a chemical that, when administered to humans, produces an effect equal to that produced in test animals by a smaller dose.^[2] Calculating the HED is a step in carrying out a clinical trial of a pharmaceutical drug.^[3]

The concept of human-equivalent energy (H-e) assists in understanding of energy flows in physical and biological systems by expressing energy units in human terms: it provides a “feel” for the use of a given amount of energy by expressing it in terms of the relative quantity of energy needed for human metabolism,^[4] assuming an average human energy expenditure of 12,500 kJ per day and a basal metabolic rate of 80 watts.^[5] A light bulb running at 100 watts is running at 1.25 human equivalents (100/80), i.e. 1.25 H-e. On the other hand, a human may generate as much as 1,000 watts for a task lasting a few minutes, or even more for a task of a few seconds' duration, while climbing a flight of stairs may represent work at a rate of about 200 watts.^[6]

The ages of domestic cats and dogs are often referred to in terms of "cat years" or "dog years", representing a conversion to human-equivalent years. One formula for cat years is based on a cat reaching maturity in approximately 1 year, which could be seen as 16 in human terms, then adding about 4 years for every year the cat ages. A 5-year-old cat would then be (5 − 1) × 4 + 16 = 32 "cat years" (i.e. human-equivalent years), and a 10-year-old cat (10 − 1) × 4 + 16 = 52 in human terms.^[7]

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