Drip Line Run Time From a Bucket Test

Garden By ZIP · 2026-10-21

A drip timer asks for minutes. Your plan for a raised bed is usually stated as a depth of water, such as half an inch for one session, or as a volume, such as ten gallons. The packaging on drip tubing gives a nominal emitter rating, but the line in your garden runs at whatever pressure your spigot, filter, regulator, and tubing length actually deliver. The reliable bridge between a stated depth and a timer setting is a short catch test: measure what a few emitters really put out, convert that to a bed-wide flow, and divide.

This guide walks that arithmetic with one worked example in customary units and a short metric version. It does not tell you how much water your crop needs. The target depth is an input you choose from your own soil checks and local guidance; the worksheet only turns that choice into minutes and shows where the conversion can mislead you. For building the watering routine itself, see the raised-bed watering guide and the seasonal watering schedule.

The one conversion you need

Depth over an area is a volume. One inch of water spread over one square foot is 12 in × 12 in × 1 in = 144 cubic inches. A U.S. liquid gallon is defined as 231 cubic inches, so 144 ÷ 231 ≈ 0.623 gallons per square foot per inch of depth. The NIST conversion-factor appendix lists the gallon-to-cubic-inch and gallon-to-liter relationships if you want to check the figures.

Metric is simpler: 1 millimeter of water over 1 square meter is exactly 1 liter. A 12 mm target on a 2.88 m² bed is 34.56 liters with no further conversion. The NIST SI volume page explains the liter and cubic-meter relationship.

Write the formula once on your bed card:

Customary: gallons = bed area (sq ft) × target depth (in) × 0.623
Metric: liters = bed area (m²) × target depth (mm)
Run time: minutes = volume ÷ measured bed flow per hour × 60

Everything else in this guide is about getting an honest number for "measured bed flow."

Running the catch test

You need a measuring cup or a kitchen jug marked in milliliters, a few small containers that fit under an emitter, and a phone timer. Run the system for a few minutes first so air is out of the tubing and the pressure has settled. Then:

Pick at least four emitters: two near the supply end of the line and two near the far end. If the bed has two parallel lines, sample both.
Slide a container under each one at the same moment and start the timer.
Collect for a fixed interval long enough to read easily. Ten minutes works for most home emitters; a shorter interval makes small reading errors a larger share of the result.
Pour each catch into the measuring cup and write down every reading separately. Do not average in your head before writing.

Run the test with the same source and the same other zones open or closed as on a normal watering day. A hose bib that also feeds a sprinkler elsewhere will deliver different pressure when that sprinkler is running.

Worked example (stated assumptions)

Illustrative example using explicitly stated assumptions; not a field test, product specification, or crop water recommendation. Assumptions: a 4 ft × 8 ft raised bed (32 sq ft); two drip lines running the 8 ft length with emitters at 12 in spacing, counted as 8 emitters per line and 16 in total; a gardener-chosen target of 0.5 in for one session; four hypothetical 10-minute catches of 610, 640, 580, and 650 mL.

Step	Reproducible result
Target volume	32 × 0.5 × 0.623 ≈ 9.97 gal (≈ 37.8 L)
Average 10-minute catch	(610 + 640 + 580 + 650) ÷ 4 = 620 mL
Average emitter flow	620 mL × 6 = 3.72 L/h ≈ 0.98 gal/h
Bed flow (16 emitters)	16 × 3.72 = 59.52 L/h ≈ 15.7 gal/h
Run time from average	37.8 ÷ 59.52 × 60 ≈ 38 minutes
Run time for lowest emitter	38.06 × 620 ÷ 580 ≈ 41 minutes

Arithmetic check: 32 square feet times half an inch is 16 "square-foot inches," and 16 × 0.6234 ≈ 9.97 gallons. One gallon is about 3.785 liters, so that is about 37.76 liters. A 10-minute catch multiplied by 6 gives an hourly rate, so 620 mL in ten minutes is 3,720 mL, or 3.72 liters, per hour. Sixteen emitters at that rate deliver 59.52 liters per hour, and 37.76 ÷ 59.52 = 0.634 hours, which is about 38 minutes.

Notice that the measured average, about 0.98 gallons per hour, is close to a common "1 gph" label but not identical. In this hypothetical the difference is small. In a real garden it can be larger, especially on long runs, on hose-end setups without a pressure regulator, or with emitters that are partly clogged. That is the reason to measure rather than read the bag.

The metric version of the same bed is about 1.2 m × 2.4 m, or 2.88 m². A 12 mm target is 34.56 liters, and 34.56 ÷ 59.52 × 60 ≈ 35 minutes. The two answers differ slightly because 12 mm is a little less than half an inch (12.7 mm) and the bed dimensions are rounded; neither is wrong, they are two different stated targets.

Average or lowest emitter: decide before you set the timer

The four catches did not agree. The lowest was 580 mL and the average was 620 mL, so the weakest sampled emitter runs at 580 ÷ 620 ≈ 0.94 of the average. If you set the timer from the average, the area around the weakest emitter receives about 94% of the target. If you set it from the weakest emitter, every sampled spot gets at least the target and the stronger ones get more.

Neither choice is automatically right. Basing the run time on the average treats the bed as one unit and accepts small differences. Basing it on the lowest emitter protects the driest spot at the cost of extra water elsewhere. Write down which rule you are using. For this example, the gap is about three minutes, 38 versus 41.

The spread itself is also information. If one emitter catches far less than its neighbors, the right fix is usually cleaning or replacing that emitter, flushing the line, or checking the filter, not lengthening the whole program to compensate. Suppose the two far-end emitters had caught 540 mL each instead: setting the timer for them would require about 38.06 × 620 ÷ 540 ≈ 44 minutes, a sign that pressure drop along the run deserves attention before the schedule does.

What the depth figure does and does not mean

Drip irrigation does not spread water evenly across the bed surface the way rain does. It wets zones around each emitter, and how wide those zones become depends on soil texture, how dry the soil was, and how long the line runs. The "half an inch" in this worksheet is therefore a bed-average equivalent: the same total volume that half an inch of rain would deliver over 32 square feet. It is a bookkeeping unit for comparing sessions, not a promise that every square inch received half an inch.

Because of that, the run time is a starting setting, not the end of the decision. After the first run, dig a narrow inspection hole or use a probe between two emitters and at the midpoint between lines. If the soil between lines is still dry at root depth, the options are a longer, slower run, an extra line, or closer emitter spacing; the arithmetic does not choose among them for you. If water is pooling or running off the bed edge, split the volume into two shorter runs separated by a pause rather than one long run.

Rain gauges and weather apps report rainfall as depth, which is the same unit as the worksheet target. That makes it easy to record "0.3 in rain on Tuesday" next to "0.5 in drip on Thursday" on one card, as long as you remember the drip figure is an equivalent spread over the whole bed.

When to repeat the test

Repeat the catch test whenever something upstream changes: a new pressure regulator or filter, a longer supply hose, an added zone on the same faucet, a different water source, or a season-start reassembly. Repeat it if plants near one end of the line consistently look different from plants at the other end. Keep the old readings on the card; a later test that is noticeably lower than an earlier one on the same setup often points to clogging.

When you add or remove emitters, recount them before reusing the bed-flow figure. The per-emitter rate may stay the same while the bed flow changes, and the timer minutes scale with it. Removing four emitters from the example bed leaves 12, so the bed flow drops to 12 × 3.72 = 44.64 L/h and the same 37.76-liter target takes about 51 minutes.

A reusable bed card

Copy these lines for each bed and fill them in:

1. Bed area and how it was measured.
2. Target depth or volume for one session, and where that choice came from.
3. Emitter count, line count, and spacing.
4. Catch-test date, interval, and every individual reading.
5. Average and lowest per-emitter flow, converted to an hourly rate.
6. Run-time rule (average or lowest) and the resulting minutes.
7. Post-run soil check result and any change made.

For measuring irregular or tiered beds before you compute area, see how to measure a raised bed. Mulch changes how quickly the surface dries but does not change the volume delivered; the mulch depth guide covers that separately.

Assumptions and limits

All bed dimensions, emitter counts, catches, and the target depth in the example are hypothetical. This guide does not recommend a watering amount, frequency, or crop-specific schedule, and it does not evaluate drip products. Emitter output depends on pressure, temperature, tubing length, elevation change, and maintenance; the catch test measures your setup at one moment under one set of conditions. Soil type, bed depth, weather, and plant size determine whether the target depth is appropriate, and the post-run soil check is the evidence that should change it. USDA hardiness zones and frost dates are not watering inputs.

Sources and method

NIST: conversion factors (volume units) — retrieved 2026-10-21
NIST: SI units of volume — retrieved 2026-10-21

AI-assisted writing with automated source, calculation and editorial checks. No hands-on testing or professional certification is claimed. Check the stated assumptions before applying this guide.