Unless you have a 17-inch laptop it is unlikely your machine will have its own numeric keypad, so this external calculator from Genius is a possible replacement.
The buttons are large and the 12-character screen is legible. Connecting to your laptop using a retractable USB cable, it has its own battery so can also be used independently.
Along with all the standard function keys of the calculator there is a button that allows you to send data directly to the Calculator program, or even Word documents, onscreen.
This is where the problems start, as you'll find you can enter numbers, but if you make a mistake you can't correct it, or even start a fresh calculation without wiping the data off the screen. This seems a little laborious and detracts from the usability of the device.