Pool Shock Calculator

For maintenance shocking, use 1 lb of cal-hypo (68–73% available chlorine) per 10,000 gallons to raise FC by about 5–7 ppm. For algae treatment, use 2–3 lbs per 10,000 gallons to reach SLAM level. Always use the calculator above for a precise dose based on your current FC, CYA, and pool volume.

Cal-hypo (calcium hypochlorite) contains 68–73% available chlorine and raises calcium hardness slightly. It does not add CYA. It is the most common and economical shock. Dichlor shock contains ~56% available chlorine and adds CYA stabilizer with every dose. If your CYA is already elevated, always use cal-hypo or liquid chlorine for shocking, never dichlor.

Shock for algae at dusk or after sunset so the high chlorine level has the full night to work without UV degradation. Brush all pool surfaces before shocking to break up biofilm. Maintain chlorine at SLAM level (roughly 40% of your CYA value) continuously until the water clears. Retest and re-dose every 4–6 hours as needed.

Wait until free chlorine drops back to a safe level before swimming. The safe swimming threshold is 5 ppm or below, though 1–3 ppm is ideal. With proper turnover this typically takes 8–24 hours depending on pool size, sunlight, and how much shock was used. Always test before allowing swimmers back in.

Weekly shocking is common practice but not always necessary if you maintain consistent free chlorine levels. Shock when: FC is depleted, after heavy bather load or a storm, at pool opening, after an algae outbreak, or when combined chlorine (chloramines) exceeds 0.5 ppm. A well-maintained pool with stable FC may only need shocking monthly.