Spring & Neap Tide Calculator

Because the tidal bulge is symmetric: the moon raises water on both the near AND far sides of Earth (differential gravity), so the sun's contribution aligns with the lunar bulge whether the two bodies are on the same side (new) or opposite sides (full). Twice a lunar month the forces stack — springs; at quadrature they partially cancel — neaps. Range ratio between them runs 1.4–2.5 depending on coastal geometry.

The lag between the astronomical lineup and the oceans' response — typically about 1.5 days (this tool's default): the biggest tides arrive a day or two AFTER full/new moon, because the ocean is a driven oscillator with inertia, not an instant mirror of the forcing. Local geography tunes the lag; old pilot books list it per port. Forgetting it puts your 'spring tide raid' on the wrong day.

Springs: crossing a shallow bar at HIGH water (maximum depth gift), refloating a grounding (more lift), and white-knuckle currents through passes (plan slack times carefully). Neaps: anchoring near drying banks (less depth swing, less scope arithmetic), working drying moorings (boat stays afloat), and any passage where strong streams hurt. The drying-out trap is the classic: anchor at neap high water, and the NEXT spring low — a week later — may leave you in mud the chart promised was water.

It's the astronomical envelope only — real ranges add local resonances, weather surge and seasonal (perigean 'king tide') modulation when the moon's also at its closest. Use it for the planning rhythm (which week to schedule the shallow harbor) and the tables/apps for the day's numbers. The cycle's SHAPE is universal physics; the amplitudes are your port's fingerprint.