Marsh Funnel Viscosity Test: Procedure, 26-Second Water Calibration and Typical Readings

The Marsh funnel times how long 946 mL (one US quart) of drilling fluid takes to drain, and fresh water at 70 +/- 5 degF (21 +/- 3 degC) drains in 26 +/- 0.5 seconds (API RP 13B-1; IADC). It is the quickest rheology check at the pit or the flow line, and a crew can run it in under a minute with a cone, a cup, a stopwatch and a thermometer. The same drilling fluid can then be compared with its own last reading.

The result is a time in seconds per quart (s/qt), not a viscosity in centipoise. Crews read it as a trend against earlier samples and against the rotational viscometer, and an equation converts it to an effective viscosity when the mud density is known.

This page gives the standard and edition, the funnel dimensions, the six steps of the test, the reporting rule, the conversion to centipoise with a worked example, the 26 s/qt water check, published readings for water wells and horizontal directional drilling (HDD), and the errors that spoil a reading.

Standards and regulations are checked against their issuing bodies, product values against the named data sheet and figures against the dated source named in the sentence; see our methodology and fact-checking.

What Is Marsh Funnel Viscosity?#

Marsh funnel viscosity, also called funnel viscosity, is the time in seconds that one quart (946 mL) of a drilling fluid takes to flow out of a Marsh funnel, reported in seconds per quart (s/qt).

A thick fluid drains slowly and a thin fluid drains fast, so the reading follows thickening or thinning of the mud from one check to the next. It does not describe the full flow curve of the fluid. That curve, and the fluid's yield stress and shear thinning, come from drilling fluid rheology measured on a viscometer, which also answers the question of what the viscosity of a mud is at a given shear rate.

Why is funnel viscosity a time and not a viscosity?#

Funnel viscosity is a flow time, not a viscosity in centipoise, because the result is reported in s/qt and needs an equation to become cP. The conversion below shows the step.

Density enters the equation, so the effective viscosity of a mud is its density multiplied by the funnel time above 25 s/qt. Two muds with the same reading in s/qt therefore differ in effective viscosity when their densities differ, and the funnel time alone does not rank them.

Which API Standard Covers the Marsh Funnel Test?#

API RP 13B-1 (6th edition, January 2026) describes the Marsh funnel test for water-based drilling fluids, and API RP 13B-2 (6th edition, July 2023, with Addendum 1, 2025, and Errata 1, 2025) is the field-testing practice for nonaqueous fluids.

Both documents are Recommended Practices, not Specifications. ISO 10414-1 is the ISO field-testing part for water-based fluids. Every field test in these documents is listed under drilling fluid testing, together with the laboratory tests.

The table compares the documents that govern field testing of drilling fluids, with their editions as of the API Standards Plan.

Marsh Funnel Apparatus: Cone, Orifice, Screen and Cup#

A Marsh funnel is a cone 12.0 in (305 mm) long and 6.0 in (152 mm) across the top that holds 1,500 mL up to its screen.

The cone drains through an orifice tube 2.0 in (50.8 mm) long with a 3/16 in (4.7 mm) bore (API RP 13B-1, per the ballot draft, confirm against the published edition).

A No. 12 (1.68 mm) screen sits 0.75 in (19.0 mm) below the rim and keeps cuttings and lumps out of the orifice. The kit adds a cup marked at 946 mL (1 qt), a stopwatch and a thermometer.

The table lists the funnel dimensions. API RP 13B-1 apparatus values, per the ballot draft, confirm against the published edition.

The metric 1,000 mL funnel variant#

Metric funnels time 1,000 mL instead of 946.3 mL, so their seconds are not s/qt (The Driller, 2022).

A report from a metric funnel names the 1,000 mL volume. The 26 s water check applies to the quart funnel only.

How to Run the Marsh Funnel Test Step by Step#

The Marsh funnel test takes one fresh mud sample, one filling to the screen and one timed drain of 946 mL into the cup, and the steps are listed below.

1. Hold the funnel upright and close the orifice with a finger.
2. Pour the sample through the screen until the mud reaches the screen, which is 1,500 mL.
3. Release the orifice and start the stopwatch.
4. Stop the watch when the mud reaches the 946 mL (1 qt) mark of the cup.
5. Measure the sample temperature in degF (degC).
6. Record the seconds to the nearest second with the temperature.

The volumes and the reporting rule are API RP 13B-1 values, per the ballot draft, confirm against the published edition. The same steps calibrate the funnel with fresh water, and a crew runs them on a fresh sample taken at the same point of the circulating system each time.

How Is Marsh Funnel Viscosity Reported?#

Marsh funnel viscosity is reported in seconds per quart to the nearest second together with the sample temperature, for example 45 s/qt at 120 degF (49 degC) (API RP 13B-1, per the ballot draft, confirm against the published edition). The reading goes on the daily mud report next to density and viscometer readings.

A reading from a metric funnel names its 1,000 mL basis, and no conversion between the two volumes is made.

Converting Marsh funnel seconds to centipoise#

Effective viscosity in cP equals the density in g/cm3 times the funnel time in s/qt minus 25 (Pitt, SPE Drilling & Completion, 2000).

For a 10.0 lb/gal (1.20 sg; 1,200 kg/m3) mud at 45 s/qt the effective viscosity is 1.20 x (45 - 25) = 24 cP (24 mPa.s). The water check gives 1.00 x (26 - 25) = 1 cP, which is the viscosity of fresh water.

The table gives effective viscosity calculated from funnel time and density, valid below about 22.5 lb/bbl (64.2 kg/m3) solids (The Driller, 2022).

The density input comes from the mud balance test run on the same sample, and the equation holds only below the solids limit explained in the section on viscometer divergence below.

What Is the Funnel Viscosity of Water? The 26-Second Calibration#

The funnel viscosity of fresh water is 26 +/- 0.5 seconds per quart at 70 +/- 5 degF (21 +/- 3 degC) (API RP 13B-1; IADC).

A funnel that drains 946 mL of water outside that window is out of calibration.

To check a funnel, fill it to the screen with fresh water at that temperature, time the quart into the cup and compare the result with the window before a test series. A funnel that misses the window is cleaned or replaced before any mud is timed. The water reading is 26 s/qt, and a mud reading is read against it.

Typical Marsh Funnel Readings by Application#

Published Marsh funnel readings range from 26 s/qt for water to 50 to 90 s/qt for HDD and well fluids in coarse soils (The Driller), and each target belongs to a fluid and ground, not to the test.

Readings in the table are field targets and field values from trade journals, not API requirements. Foundation support fluids follow US DOT slurry tables that are kept with that application.

The table collects published Marsh funnel readings by fluid and ground; the targets are not API requirements.

Checking a fresh bentonite mix with the funnel#

Fresh bentonite mud for water wells starts near 35 s/qt against 26 s/qt for water (Water Well Journal).

Rerun the funnel after the clay has hydrated and after each addition, and record each reading with its temperature. The dose that moves the reading depends on the grade of bentonite and on the water.

Common Marsh Funnel Errors and Limits#

Five errors spoil most Marsh funnel readings: a funnel out of calibration, a missing or blocked screen, the wrong volume, an unrecorded temperature and a reading compared across funnels of different volume.

• Funnel out of calibration: check 26 +/- 0.5 s with fresh water.
• Screen missing or blocked: refit and clean it.
• Wrong volume: the quart funnel times 946 mL and the metric funnel times 1,000 mL.
• Temperature not recorded: record degF (degC) with each reading.
• Readings from funnels of different volume compared: compare like for like.

The funnel gives one number per sample, so it is read as a trend over successive checks and not as a property of the fluid on its own.

Why funnel readings stop tracking viscometer readings#

Effective viscosity from the funnel follows rotational viscometer readings only up to about 22.5 lb/bbl (64.2 kg/m3) solids in one published test series, and the limit varies by product (The Driller, 2022).

Above that solids level a crew relies on rotational viscometer readings instead of the converted funnel value. The 6 and 3 r/min dial values and the gel strengths are read only on the viscometer.

Marsh Funnel or Rotational Viscometer: Which Check Answers What?#

The Marsh funnel answers whether a fluid has thickened or thinned since the last check, while a rotational viscometer answers why, through dial readings at 600, 300, 6 and 3 r/min.

Both tools are field tests of the API Recommended Practices, and the funnel is checked with water alone. A reading from either one is recorded next to the sample temperature. Flow-cone tests for cement grouts are a separate field.

Plastic viscosity is the 600 r/min reading minus the 300 r/min reading, and yield point is the 300 r/min reading minus the plastic viscosity, so plastic viscosity and yield point never come from the funnel.

Funnel checks in HDD, water-well and foundation drilling#

Three drilling applications run the Marsh funnel as their main pit check, listed below.

• HDD drilling fluids are checked against targets by ground.
• Water well drilling mud starts near 35 s/qt and rises for lost-circulation pills.
• Bentonite and polymer support fluids follow US DOT slurry tables that include the Marsh test.

Contractors who want the fluid checked on site can describe the bore to independent drilling-fluid service providers.