Handy calculators: antenna lengths, cable loss, beam heading and distance, Maidenhead locator and SWR.
Wire antennas for a frequency
Cut a little long and trim for the lowest SWR – height, nearby objects and the wire insulation all change the length a bit. An EFHW also needs a 49:1 (or 64:1) transformer.
How much power does the cable eat?
Typical manufacturer figures for a matched line (SWR 1:1); a high SWR adds extra loss. Old or wet cable is worse.