firpmord

[1] A low-pass filter
f = [1500 2000];        // frequency edges for bands
a = [1 0];              // desired amplitude for each band
dev = [0.01 0.1];       // Acceptable deviation for each band
fs = 8000;              // Sampling frequency
[n,fo,ao,w] = firpmord(f,a,dev,fs);

[2] A bandstop filter
f = [1000 1800 2400 3000];
a = [1 0 0.5];
dev = [0.01 0.1 0.03];
fs = 8000;
[n,fo,ao,w] = firpmord(f,a,dev,fs);

References
[1] Rabiner, Lawrence R., and Bernard Gold. "Theory and application of
digital signal processing." Englewood Cliffs, NJ, Prentice-Hall, Inc.,
1975. 777 p. 156-7 (1975).
[2] Rabiner, Lawrence R., and Otto Herrmann. "The predictability of certain
optimum finite-impulse-response digital filters." Circuit Theory,
IEEE Transactions on 20.4 (1973): 401-408.