For non pumpers and pumpers see this page http://www.diabetesnet.com/diabetes_control_tips/bolus_on_board.php

The Bolus on Board tells how many units of rapid insulin are still working. This helps in deciding whether more insulin or more carbohydrate is needed. Never assume you need more insulin simply because your blood sugar is high at the moment. Always determine how much bolus is on board before taking more. If your blood sugar is normal with bolus on board, you may need to decide if carbohydrate is required to keep it normal.

Rapid insulins are commonly perceived as faster than they really are. The kinetics of an insulin are when that insulin can be measured in the bloodstream, whereas an insulin’s dynamics are the time over which it will actually lower the blood sugar. After an injection, peak levels of Humalog and Novolog insulins are seen in the bloodstream about 45 minutes later, as shown by the dashed kinetic line to the right. The maximum effect on lowering the glucose level is not seen until 2 hours after an injection or bolus and continues for over 4 to 5 hours.

For more information on when these insulins really work, see our Kinetic vs Dynamic page.

There is a power point presentation which show you how different pumps calculate the insulin on board. They dont all use the same way and are surprisingly quite different.

bolus on board (PDF 310k)