A toroid is a long solenoid which is bend into circular form. So, toroid is equivalent to the solenoid having infinite length but it has finite no. of circular turns. Consider a toroid having radius r, carrying current I through it. If N is the number of turns in toroid and n be the no. of turns per unit length then,

again magnetic field due to infinite solenoid is,

then the magnetic field due to toroid is,

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