Planimeters didn't calculate in the sense of allowing a user to enter some numbers and producing a result. They did, however, solve a tedious and common problem - that of determining the area of some closed shape. In other words these devices were integrating machines.
To use a planimeter, the user set up the device, zeroed the recording wheels and then traced the pointer all the way around a closed shape (typically a clockwise direction was used to produce a positive result.) The area could then be read directly off the dials. Better planimeters allowed adjustments to allow the device to read in various units and scales. Other units simply read in a fixed scale such as square centimeters and required the results to be scaled to the units required.
Planimeters were made in a number of ways. The devices shown below are Polar Planimeters. To use them, the user set pole weight on the desk usually outside the area to be measured. The pole weight sometimes had one or more pins in the bottom to make it stay put. The pole arm connected the pole weight to the carriage and the tracer arm connected the tracer point to the carriage. This caused the carriage to always move around a semi-circle regardless of the shape the tracer point followed.
Underneath the carriage was a measuring wheel. While the carriage always followed a circle, the angle of the tracer arm caused the measuring wheel to be anywhere between tangential to the circle and
measuring wheel
carriage