Tree Watering Calculator
Weekly tree water from trunk diameter.
What it does
Established trees drink by the trunk. Ten gallons a week per inch of diameter keeps roots charged through dry spells: twelve inches wants roughly one hundred twenty gallons. Slow ring soaking beats trunk spraying every time.
The tool above multiplies diameter out. Defaults give 120 gal weekly for twelve inches. The metric mirror at thirty centimeters gives 447 liters. A six inch youngster wants sixty gallons, showing how fast needs scale with girth.
Rain resets the clock. Good storms skip the week while mulch stretches every gallon further. New transplants flip the script with frequent smaller help until roots run beyond the ball.
This page plans established tree water only. It does not schedule new plantings, diagnose disease, or test soil. Arborists grade stressed trees on site.
How to use
- Pick imperial (in) or metric (cm).
- Measure trunk diameter at chest height.
- Read weekly water first.
- Read the rule beside.
- Ring soak at the drip line.
- Skip weeks with good rain.
Formula
- DBH
- Trunk diameter at 4.5 ft height.
- 10 gal
- Ten gallons per inch each dry week.
- 12 in
- Soak target depth.
Established tree rule of thumb in dry spells. At zero diameter there is nothing to water. Rain, mulch depth, soil, and species sit outside this line.
Worked example
Example: 12 in trunk
Defaults: imperial 12 in DBH. Metric mirror is 30 cm. DBH means chest height.
- Inputs (imperial default): 12 in trunk diameter.
- Water: 12 times 10 is 120 gal weekly.
- Rule: 10 gal per inch DBH.
- Metric mirror (30 cm) gives 447 L weekly.
- Soak slowly to 12 in deep in dry spells. Tool shows the same.
Reference table
| Trunk | Weekly | Metric |
|---|---|---|
| 12 in | 120 gal weekly | 454 L |
| 6 in | 60 gal weekly | 227 L |
| 30 cm | 118.1 gal | 447 L weekly |
| 20 cm | 79 gal | 298 L weekly |
| 24 in | 240 gal weekly | 908 L weekly |
Limits
- Rule of thumb. Rain excluded.
- Established trees. Transplants differ.
- No soil or species model.
- Dry spells assumed.
Common mistakes
- Spraying trunks instead of drip rings.
- Daily sprinkles growing shallow roots.
- Watering through good rain weeks.
- Ignoring mulch that halves need.
- Missing new tree schedules.
Try it yourself
Quick checks. Answers sit tucked under each question.
30 cm trunk?
Trunk is 11.8 in DBH. Water is 118.1 gal, or 447 L weekly.
6 in trunk?
Water is 6 times 10 is 60 gal weekly.
Questions
How is tree water figured?
DBH inches times 10 gallons weekly. Here 120 gal weekly.
What is DBH?
Diameter at breast height, about 4.5 ft up the trunk.
Does this fit new trees?
New transplants need frequent shallow help until roots run. This rule fits established trees.
Where do I put water?
Yes. Drip ring at the drip line beats trunk spraying.
Is this exact weekly?
No. Rain, soil, mulch, and species shift real need week to week.
Deep or daily watering?
Deep infrequent soakings grow deep roots. Daily sprinkles grow shallow ones.