LittlePixel

The thing about calculating spoke length is it's affected by several different factors of size, which you haven't given us;

Firstly - the rim may be 16" but this is a sort of arbitary measurement used in the industry and is likely one of two distinct sizes - ISO 305 and ISO 349 (this is the height of the rim from top to bottom in millimetres). When you measure the rims for the purposes of spoke length calculation, you need to measure the INSIDE size of the rim, not the outside

Secondly - the size of your hub affects the calculation too - both the latitudinal width between the two flanges and the radial distance of the holes from the centre of the axle. Not to mention whether the hub is centred (most geared cassette wheels have offset flanges to fit the gears, meaning spokes on the drive side are shorter than on the non-drive side)

When getting spokes, you really need to have the exact rims and hubs you are going to use in your hands so you can take the appropriate measurements accurately before you use a calculator to find the size you need.

I know this isn't maybe the full answer you hoped for - but the answer really is - get the parts you are using, take the time to make some accurate measurements (using a vernier caliper if possible) so thast using a calculator should throw out the length of spoke you need with no trouble at all.

Failing that- take the rims and hubs to someone that can. Wheelbuilding isn't that expensive and most local bike shops will do it, though some will be thrown at the idea of building small wheels.

Huw