U.S. Navy circumference method — no calipers needed.
This calculator uses the U.S. Navy circumference method, which estimates body fat percentage from waist, neck, and (for women) hip measurements. Studies show it to be accurate within 3–4% compared to DEXA scans for most people.
Measure at the narrowest point for waist, around the Adam's apple for neck, and at the widest point for hips. Take measurements in the morning for best consistency.