Parts per Trillion in Drinking Water: ppt and ng/L

By SubcontractorHub Editorial Team·Published October 2026

Parts per trillion in drinking water is an information page about the unit, not a contaminant guide. The public PFAS lines that use this unit are PFAS in well water. The step between milligrams per liter and micrograms per liter is milligrams and micrograms. This page starts one step smaller than that pair. It does not set a limit.

Why the Lab Prints Two Names for One Number

A certificate can say ppt, ng/L, or both. Dealers lose the conversation when one person reads the letters and another reads the digits as if they were milligrams. The fetched EPA PFAS table, on the PFOA row, prints the MCL as 4.0 parts per trillion and says that unit is also expressed as ng/L. Virginia's certificate section says results are generally listed as parts per trillion or ppt, and that this is notated as ng/L, nanograms per liter.

Those two sentences are the whole conversion between ppt and ng/L on the sources used here. There is no multiplier. 4.0 ppt is 4.0 ng/L. Writing both is a label, not a calculation. If a meter or a lab uses a different convention, do not force this pair onto it.

The Thousand-Fold Step Virginia Prints

Virginia says the laboratory might report results as µg/L, micrograms per liter or parts per billion. If so, move the decimal point three units to the right to convert to ng/L. The example on that page is 1 µg/L equals 1,000 ng/L. Three decimal places is the same thousand used when a milligram becomes a thousand micrograms. It is the next step down, not a new chemistry.

The 0.004 µg/L row is arithmetic on Virginia's 1,000, so a reader can see 4 ng/L without guessing. It is not an extra regulatory line. The public 4.0 ppt figure, and whether it applies to a private well, stays on the PFAS page. The tool that multiplies and divides by 1,000 between micrograms and milligrams is the microgram milligram converter. It does not output nanograms. Use it only when the sheet is already in µg/L or mg/L.

What Parts per Trillion in Drinking Water Is Not

It is not parts per million. Many inorganic MCLs are in milligrams per liter, and the primary table's older note, used on other pages here, treats milligrams per liter as parts per million. A digit of 4 in mg/L is not a digit of 4 in ppt. Fluoride's public 4.0 mg/L line is a different 4.0 from PFOA's 4.0 ppt. Say the unit out loud before you compare them.

It is not picocuries. Activity units for radionuclides stay on the picocurie becquerel converter. It is not a hardness grain. It is not a removal percentage. Virginia also offers a picture of how small a nanogram is. That picture is not used as math on this page.

How to Read the Certificate Without Moving the Wrong Way

1. Copy the unit exactly: ppt, ng/L, µg/L, ppb, or mg/L.
2. If the unit is ppt or ng/L, keep the digits. Do not divide or multiply.
3. If the unit is µg/L, and you need ng/L, multiply by 1,000, which is Virginia's three-place move to the right.
4. If the unit is mg/L, you are on the milligram page, not this one, until you have moved to micrograms first.
5. Write the public comparison, if you make one, with both the number and the unit. A bare 4.0 is not a result.

The two public columns those comparisons use are MCL and MCLG. Which table is enforceable for a public system, and which is aesthetic, is primary and secondary drinking water standards. A single-home well is still outside that statute, as private well water regulations describes.

What Should You Write Next to a ppt Result?

Write the compound name, the digits, the unit as printed, the unit you converted to if you converted, and the factor 1 or 1,000. Write whether you compared the number with a public line, and the date of the page you used for that line. A conversion without the compound name is how PFOA and a different PFAS get swapped.

Sample point still matters. A raw-well ppt and a treated-tap ppt are two results. The well water testing guide is where the panel is ordered. The job file for that sheet can sit with the rest of the dealer's work on water treatment contractor software. This explainer does not choose a device.

Who Should Not Use the Decimal as a Verdict

Moving 1 µg/L to 1,000 ng/L does not say the water passed or failed. It does not say a filter will change the number. It does not turn a CDC screening level into an MCL. If the sheet uses a method name, copy the method. EPA methods for drinking-water PFAS are discussed on the PFAS page from the state note, not recalculated here. When the unit is already clear, stop converting and talk about the compound.

Write the Factor Next to the Digit

A converted number without the factor is how a later reader undoes the work. If you kept the digits, write 1. If you moved micrograms to nanograms, write 1,000. If you did not convert, say so. The compound name stays on the same line, because a unit conversion does not identify which PFAS the lab measured.

Compare a public line only after both numbers wear the same unit. A 4.0 in milligrams per liter and a 4.0 in nanograms per liter are not the same result, and they are not the same rule. The public lines themselves, including which of them are proposals, stay on the PFAS page. This explainer stops at the unit.

Frequently Asked Questions