Toroid calculator
Toroid inductance and flux estimator
Pick a popular ferrite or iron powder core and compute inductance, reactance and flux density.
Flux density is approximate; stay below the listed maximum to avoid saturation.
Inductance on a toroid comes from the core's Aℓ (AL) constant: L = AL · N². Once you pick a material, permeability and core geometry are baked into AL, so increasing turns has a squared effect on inductance.
Use the flux density and power estimates to judge suitability. A mix 31 ferrite core run at 1 A on 7 MHz may approach its 0.3 T ceiling—add turns to reduce flux, or switch to a higher power mix like powdered iron #2. Always wind evenly around the circumference to avoid hot spots and keep RF resistance low.