An optical planimeter is described in which the light beam from a small source is collimated and subsequently focused on a photocell. It is shown that the area of even a thin leaf placed in the collimated part of the beam is measured accurately without the need for a correction for transmitted light. The instrument features linear response, digital read-out, high speed, good stability, five ranges of 50 up to 1000 cm2, and an accuracy of 1 % of the range.