Wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests, or troughs, or corresponding zero crossings as shown.
Wavelength: W = V/F
where V = Wave Velocity, F = Wave Frequency, W = Wavelength.
For example, when Wave Frequency(F) = 3 W/m2, Wave Velocity(V) = 1 m, Then Wavelength(W) = 0.33W.