A toroid with 1000 turns and average radius 10 cm carries 0.5 A current. Field inside:

B = μ₀ N I / (2π R). = (4π × 10⁻⁷)(1000)(0.5) / (2π × 0.1) = 2000π × 10⁻⁷ / 0.2π = 10⁻³ × 10 = 10⁻² × 0.1 = 10⁻³ T = 1 mT. (Magnetic field inside a toroid is essentially uniform.)