Lab calculations: sample weight, loss on ignition and loss on drying

In short: The sample weight is the mass of sample a result refers to. Loss on drying is the mass lost during drying, loss on ignition the mass the dried sample loses during ignition. All three values come from weighings. What matters is that every weighing is recorded cleanly and the reference basis (wet or dry) is clear.

Why simple lab calculations still go wrong

Sample weight, loss on drying and loss on ignition are simple calculations. Errors rarely sit in the formula. They sit in the workflow around it:

• Tare forgotten: The mass of the crucible or dish is not subtracted.
• Reference basis mixed up: A value is related to the wet mass instead of the dry mass, or the other way round.
• Transcription errors: Weighing results are copied by hand into a spreadsheet.
• No traceability: Later, nobody can show which weighing led to which result.

The sections below show each formula with its standard and source. The quoted US EPA methods are freely available and contain the full calculation rules.

Sample weight: definition and calculation

The sample weight is the mass of sample you weigh out for a determination. It sits in the denominator of almost every content calculation, so an inaccurate sample weight distorts every result built on it. How precisely to weigh is set by the method. One example from the acid digestion of sediments, sludges and soils:

“For each digestion procedure, weigh to the nearest 0.01 g and transfer a 1-2 g sample (wet weight) or 1 g sample (dry weight) to a digestion vessel.”

Source: US EPA Method 3050B, Revision 2 (December 1996), Section 7.1

Content based on the sample weight

When the sample is digested and the solution is measured, EPA Method 200.7 converts the reading back to the content of the solid sample:

Sample concentration (mg/kg) = C × V × D / W

“W = Weight of sample aliquot extracted (g x 0.001 = kg)”

Source: US EPA Method 200.7, Revision 4.4 (1994), Section 12.5

Here C is the concentration in the extract (mg/L), V the extract volume (L), D the dilution factor and W the sample weight in kg. The sample weight feeds straight into the result: if it is recorded 1% too high, the calculated content is about 1% too low.

Wet or dry? The reference basis

Every result must state whether it refers to the wet sample weight or to the dry mass. EPA 3050B puts it this way:

“The concentrations determined are to be reported on the basis of the actual weight of the sample. If a dry weight analysis is desired, then the percent solids of the sample must also be provided.”

Source: US EPA Method 3050B, Section 7.6.1

The conversion to dry mass follows arithmetically: content on a dry basis = content on a wet basis / (percent solids / 100). For that you need the percent solids, which brings us to loss on drying.

Calculating loss on drying and percent solids

During drying, the sample loses water and possibly other volatile components. You weigh before and after drying. This gives two values that add up to 100%:

Percent solids (%) = m_dry / m_wet × 100

Loss on drying (%) = (m_wet − m_dry) / m_wet × 100 = 100 − percent solids (%)

The first formula is given in EPA 200.7, the second follows arithmetically:

“% solids (S) = DW / WW x 100”

Source: US EPA Method 200.7, Revision 4.4, Section 12.6 (DW = sample weight after drying, WW = sample weight before drying)

The drying temperature is set by the method. EPA 1684, for example, dries at 103 to 105 °C:

“Sample aliquots of 25-50 g are dried at 103°C to 105°C to drive off water in the sample.”

Source: US EPA Method 1684, Draft January 2001, Section 2.1

In Europe, the corresponding standard for sludge and solid environmental matrices is EN ISO 11465, published in Germany as DIN EN ISO 11465:2026-03:

“Sludge and solid environmental matrices - Determination of dry residue or water content and calculation of the dry matter fraction on a mass basis”

Source: DIN Media, product page DIN EN ISO 11465:2026-03 (English title)

Calculating loss on ignition

For the loss on ignition, the already dried sample is ignited in a muffle furnace. Mainly organic matter is driven off. EPA 1684 describes the step like this:

“The residue from Section 2.1 is cooled, weighed, and dried again at 550°C to drive off volatile solids in the sample.”

Source: US EPA Method 1684, Section 2.2

The European standard for soil, waste and sludge is EN 15935, in Germany DIN EN 15935:2021-10, titled “Soil, waste, treated biowaste and sludge - Determination of loss on ignition” (DIN Media).

EPA 1684 calls the result “volatile solids”. With m₀ = empty crucible, m_T = crucible with dried residue and m_I = crucible after ignition:

Loss on ignition (%) = (m_T − m_I) / (m_T − m₀) × 100

Residue on ignition (%) = (m_I − m₀) / (m_T − m₀) × 100

“Wvolatile=Weight of residue and dish after ignition (mg)”

Source: US EPA Method 1684, Section 12.1, Equations 3 and 4

Watch the denominator: loss on ignition refers to the dry mass, not to the wet sample weight. Mixing them up gives a clearly different value, as the example below shows.

Worked example, step by step

The numbers are example values for illustration, not measurement data.

1. Empty crucible (m₀): 25.000 g
2. Crucible with wet sample: 35.000 g. The sample weight is 10.000 g.
3. After drying (m_T): 33.000 g. Dry mass 8.000 g, percent solids 80.0%, loss on drying 20.0%.
4. After ignition (m_I): 32.200 g. Loss on ignition = 0.800 g / 8.000 g × 100 = 10.0% on a dry basis. Residue on ignition 90.0%.

On a wet basis it would be only 8.0% (0.800 g / 10.000 g). The difference of two percentage points comes from the reference basis alone. That is why it belongs with every result.

The same applies when converting a content value: 50 mg/kg on a wet basis corresponds, at 80.0% solids, to 50 / 0.80 = 62.5 mg/kg on a dry basis.

Common weighing errors

• Moisture after drying: Dried residues quickly take up water again. EPA 1684 warns: “It is imperative that dried samples be weighed quickly since residues often are very hygroscopic and rapidly absorb moisture from the air.” (Section 11.1.3)
• No constant weight: Drying or ignition is repeated until the mass barely changes. EPA 1684 sets the criterion “until the weight change is less than 4% or 50 mg, whichever is less” (Sections 11.1.4 and 11.2.2).
• Duplicates not checked: “Duplicate determinations must agree within 10% of their average.” (EPA 1684, Section 12.2)
• Procedure not documented: Temperature, duration and reference basis belong with the result. Without them a value cannot be compared.

Calculating lab values automatically in a LIMS

Calculating by hand means reading the balance, writing the value down, typing it into a spreadsheet, calculating and copying the result back. Each step is a source of error. In [FP]-LIMS it works differently:

• Formulas per method: Sample weight, loss on drying, loss on ignition and other values are calculated with stored formulas or calculated parameters per method or inspection plan.
• Weights straight from the balance: Connected balances deliver the values automatically. Where no balance is connected, you enter the values manually.
• More values: Ash content, moisture and density can be calculated the same way.
• Traceable: Every weighing and every result is linked to the sample. Changes are recorded in the audit trail.

Setting up your own formulas in [FP]-LIMS is part of the Advanced Training at the [FP]-LIMS Academy. How reliable a calculated value is also depends on its measurement uncertainty. Gravimetric values such as loss on ignition are typical for soil, sludge and waste samples. Learn more on our page about LIMS for environmental laboratories.

Frequently asked questions

What is the difference between loss on drying and loss on ignition?

Loss on drying is the mass lost during drying, related to the wet sample. Loss on ignition is the mass the already dried sample loses during subsequent ignition, related to the dry mass. EPA Method 1684 ignites at 550 °C.

What does loss on ignition refer to?

To the dry mass. The denominator of the formula is the mass of the dried residue, not the wet sample weight (US EPA Method 1684, Equation 4).

How accurate does the sample weight need to be?

That is set by the method. EPA 3050B, for example, requires weighing to the nearest 0.01 g. Check the requirement of the method your lab works to.

Can [FP]-LIMS calculate sample weight, loss on ignition and loss on drying automatically?

Yes. [FP]-LIMS calculates these values with formulas per method or inspection plan. Weights come from connected balances or are entered manually. Ash content, moisture and density can be calculated the same way.

Sources

• US EPA Method 1684: Total, Fixed, and Volatile Solids in Water, Solids, and Biosolids, EPA-821-R-01-015, Draft January 2001
• US EPA Method 200.7, Revision 4.4 (1994)
• US EPA Method 3050B, Revision 2 (December 1996)
• DIN EN 15935:2021-10, DIN Media product page
• DIN EN ISO 11465:2026-03, DIN Media product page