The key to all toroidal transformer advantages is its efficiency. And the key to its efficiency is the core. The core is a continuous strip of grain oriented silicon steel, wound like a clock spring, under tension. It is annealed to relax the molecular structure which ensures that all grains are in the magnetic direction (unlike the old standard EI, with about 40% in the wrong direction). There is no air gap, resulting in a stacking factor of 95% of its weight. Since all the windings are symmetrically spread over the entire round, gapless core, a higher flux density is possible. The magnetic flux is in the same direction as the grain oriented silicon steel core, thus achieving very high electrical efficiencies. Typical efficiency figures for toroidal transformers are 90%.

Toroidal transformers radiate about 1/10 the magnetic field of EI transformers; this is, again, because of the inherent efficiency and unique construction. The windings which cover the core act as a shield. The magnetic field is contained doing what it should, transforming energy from primary to secondary. This may eliminate the need for special shielding. Likes HIFI equipment and medical equipment.
Toroidal transformers are also acoustically quieter than EI transformers. In addition the windings tightly envelope the entire core, effectively reducing magnetostriction – the main source of the familiar mechanical hum found in standard vertically laminated EI transformers. Compared to EI transformers, toroidal are silent. That is why the toroidal transformer is suitable use in sensitive equipment.
This efficiency yields the most easily identifiable feature of the toroid, its size benefit. Toroidal transformers are about half the size and weight of standard transformers.