# Two-point calibration

> Two-point calibration sets a pH meter with two buffers, usually pH 7.00 then 4.01, fixing both its offset and its slope for accurate readings.

- Canonical: https://genetixbay.com/knowledge/glossary/two-point-calibration.html
- Also written: 2-point calibration, two point cal, dual-point calibration, slope calibration, multi-point calibration
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Two-point calibration adjusts a [pH meter](https://genetixbay.com/knowledge/glossary/ph-meter.md) using two reference buffers. The first, almost always [pH](https://genetixbay.com/knowledge/glossary/ph.md) 7.00, sets the offset (the zero point, where an ideal electrode outputs 0 mV). The second, usually pH 4.01 for acidic work, sets the slope, the millivolts per pH unit. An ideal electrode gives about 59.16 mV per pH unit at 25 °C. One-point calibration fixes only the offset and can mislead on worn probes.

## In practice

For grow solutions in the 5–7 range, calibrate at 7.00 then 4.01. Steps: rinse with distilled water, place in pH 7, let the reading settle, confirm; rinse; place in pH 4, settle, confirm. Many meters then report slope as a percentage. A slope of roughly 95–105% means the electrode is healthy; below about 90% suggests cleaning or replacing it.

Calibrate weekly for regular hydro use, and always after storage, cleaning, or a reading that [looks](https://genetixbay.com/knowledge/glossary/bag-appeal.md) wrong. A three-point calibration (adding pH 10) helps only if you measure alkaline samples.

## Units and formula

Nernst slope ≈ 59.16 mV per pH unit at 25 °C; slope % = measured slope ÷ theoretical slope × 100.

## Don't confuse with

- **EC calibration**: usually one-point, with a standard such as 1.413 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md).
- **Temperature compensation**: corrects for temperature, not for electrode ageing.

## Related terms

[Calibration solution](https://genetixbay.com/knowledge/glossary/calibration-solution.md), [pH meter](https://genetixbay.com/knowledge/glossary/ph-meter.md), [ATC](https://genetixbay.com/knowledge/glossary/atc.md)

## Read more

- [pH and EC meter calibration: buffers, slope, storage and when to replace a probe](https://genetixbay.com/knowledge/ph-ec/meter-calibration.md)