Surface area

Surface area measures how much exposed surface is available for exchange, whereas volume measures how much space is inside. In biological models, volume can represent the amount of living material that must be supplied and have its waste products removed.

As an organism gets larger, its volume grows faster than its surface area. It therefore has less outer surface available for each unit of volume. Cells in the centre are also farther from that surface, so diffusion across the outside alone cannot supply them quickly enough or remove their wastes quickly enough.

Surface area measures the outside of an object, whereas volume measures how much three-dimensional space it occupies. Volume can represent the amount of living tissue requiring supplies and producing waste. Surface area can represent the boundary available for exchange.

Surface area describes how much boundary is available for exchange; volume is related to how much living tissue needs supplies and produces waste. Comparing them gives the surface area:volume ratio.