The pitch of B-DNA is approximately 3.4 nanometers per complete turn. In this context, pitch means the distance measured along the helix’s axis from one point on the spiral to the equivalent point one full rotation later.
B-DNA is the familiar right-handed double-helix form that predominates under many cellular conditions. Its backbone forms the outside of the helix, while complementary bases pair inside: adenine with thymine and guanine with cytosine. Each base pair rises roughly 0.34 nanometers along the axis, and about ten such rises produce the classic 3.4-nanometer pitch.
A common source of confusion is that modern structural measurements often describe physiological B-DNA as having about 10.4–10.5 base pairs per turn rather than exactly 10. That makes the effective pitch vary slightly with sequence, hydration, ionic conditions, and measurement method. The textbook value remains 3.4 nm, or 34 ångströms. This is distinct from the helix’s diameter, which is roughly 2 nm, and from A-DNA and Z-DNA, which have different geometries and pitches.