EC Conversion Chart: mS/cm, µS/cm, PPM, and CF

EC, PPM, µS/cm, CF — every nutrient meter seems to speak its own language, and converting between them by hand is a fast way to make a mistake with your reservoir. This is the reference chart we point growers to whenever the units stop lining up.

"Buy a meter, learn its scale, and stick with it. Most nutrient problems I see aren't bad recipes — they're someone reading a 700-scale meter like it's a 500-scale one." — Andrew Woodward

The four units, explained

• EC (electrical conductivity) — the actual physical measurement every meter takes, in milliSiemens per centimeter (mS/cm). This is the only unit that doesn't depend on a conversion formula, which is why serious growers eventually default to it.
• µS/cm (microsiemens per centimeter) — the same measurement as EC, just expressed in smaller units. EC × 1000 = µS/cm. An EC of 1.8 mS/cm is 1800 µS/cm.
• PPM (parts per million) — an estimate, not a direct measurement. Meters calculate it from EC using one of two industry conversion factors, "500" or "700," which is exactly why the same solution can show two different PPM numbers on two different meters.
• CF (conductivity factor) — mostly seen on older or imported meters, CF = EC (mS/cm) × 10. An EC of 1.8 mS/cm reads as CF 18.

Conversion chart

EC (mS/cm)	EC (µS/cm)	CF	PPM (500 scale)	PPM (700 scale)
0.4	400	4	200	280
0.6	600	6	300	420
0.8	800	8	400	560
1.0	1000	10	500	700
1.2	1200	12	600	840
1.4	1400	14	700	980
1.6	1600	16	800	1120
1.8	1800	18	900	1260
2.0	2000	20	1000	1400
2.2	2200	22	1100	1540
2.4	2400	24	1200	1680
2.6	2600	26	1300	1820
2.8	2800	28	1400	1960
3.0	3000	30	1500	2100

The formulas, if you'd rather do the math yourself

• mS/cm → µS/cm: multiply by 1000
• mS/cm → CF: multiply by 10
• mS/cm → PPM (500 scale): multiply by 500
• mS/cm → PPM (700 scale): multiply by 700
• PPM (500 scale) → PPM (700 scale): multiply by 1.4

Which scale does your meter use?

Most US-made TDS meters (HM Digital, Bluelab's US models) use the 500 scale. Many European and Australian meters (Truncheon, some Hanna models) use the 700 scale. When in doubt, dose your reservoir to a known EC target using an actual EC meter — it removes the scale question entirely, which is one of the reasons we build our meters to read EC directly rather than only PPM.

The bottom line

PPM is a convenient shorthand, but EC is the ground truth — it's what your meter is actually sensing, and it's the number that stays consistent no matter whose chart you're reading. If you ever get a reading that doesn't match expectations, check the scale before you check your nutrient recipe.

Want to dial in your own numbers? Check our nutrient strength guide for hydroponic herbs, and measure with confidence using the Aero-Gro Pro 5-in-1 Water Meter.