The magic circle is the visible manifestation of magical energy being collected by a mage to cast a powerful spell. It also accompanies Device activation and similar power-ups. The circle is composed of intangible light and usually appears just under the caster's feet (even in flight), centered under his or her center of gravity. Depending on the amount of energy released, the circle may spin faster or slower. The circle is white with a glow depending on the caster's magic color, with the exception of Reinforce whose circle is black with a purple glow.
According to A's manga, the shape of the magic circle is determined by the particular spell being cast. Because magic in the Nanoha universe is similar to programs, each spell includes complex mathematical calculations, and the magic circles are basically the visual representation of said calculations.
The form of a mage's magic circle is determined by the magic system he or she practices:
- The Mid-Childan system uses a proper circle, with more concentric rings and squares inscribed in it. At the corners of each ring are the small Greek letters π (pi), δ (delta), ω (omega) and ν (nu).
- The Belkan systems, both Ancient and Modern, use a triangle, with four circles at its angles and the center. The outer circles contain abstract symbols, while the inner circle is inscribed with a cross. The Belkan Circle also appears in the series a few times alongside the Wolkenritter, consisting of two overlayed Belkan triangles circumscribed by a circle.
- The Inherent Skills of Combat Cyborgs use a circular IS Template which is more technological-looking than the others, composed of solid blocks of colour and no symbols.
- Thoma and other Eclipse virus carriers display a circle containing a 10-pointed star, which is presumably the circle of Eclipse spells.
- Practitioners of Formula Eltria (e.g. the Florian sisters) use a magic circle known as Formula Plate for their spells.
Within a same system, there could also be variations of magic circles:
- Summon magic, regardless of school, apparently uses a magic square based on the caster's normal magic circle.
- A "magic loop" (環状/帯状魔法陣 Kanjō/Taijō Mahōjin) appears around the caster's arms or devices in numerous spells or skills (mainly shooting or bombardment type), which is used for strengthening and accelerating the magic charge. Flat and broad versions of the loops also appear as the "floors" for Wing Road and Airliner.
- A sighting ring replaces the magic loop for magic charge in Teana's Phantom Blazer.
Although this is uncommon, an individual who has mastered more than one magic system can manifest magic circles of different form, depending on the currently used system. For example, the summoners apparently only manifest their Squares when summoning extremely powerful creatures, and use regular Circles or Triangles otherwise. When Hayate Yagami casts Hraesvelgr with aiming assistance from Long Arch it manifests with a Mid-Childan Circle rather than her usual Belkan Triangle. In the refined DVD version of A's, Hayate also has a Mid-Childan Circle under her feet and a Belkan Triangle for the muzzle in her Ragnarok spell.
In the movie continuity, the magic circles (Mid-Childan circle, Belkan triangle and magic loop so far) have been redesigned.
- Magic circle in traditional magic