Quick verdict
A pocket steel tape with standard inches on one side and diameter graduations on the other, so wrapping it around a trunk, branch or pipe reads the diameter directly to a hundredth of an inch. Metric is on both sides, the case has a magnet and a rotating wheel brake, and it retracts like a normal tape. For arborists, foresters and anyone sizing trees, fertiliser rates or tree wrap, it replaces a calculator.
Strengths
- Reads diameter directly; no dividing by pi
- Standard inches and metric on the same tape
- Diameter scale to 1/100 inch, linear to 1/16 inch
- Magnetic case and rotating brake for one-handed use
- Twelve feet covers trunks up to about 45 inches across
Weaknesses
- Half-inch width is narrow for very rough bark
- Steel tape kinks if abused on thorny trees
- Twelve feet limits it to mid-sized trunks
The overall YardToolLab Score is out of 100. Section scores are out of 10.
The Number Every Tree Job Needs
Tree fertiliser is dosed per inch of trunk diameter. Trunk guards and wraps are sized by diameter. Municipal tree rules and arborist reports use diameter at breast height. Measuring circumference and dividing by pi works, but a diameter tape reads the answer directly. Perfect Pi’s tape is a 12 foot pocket steel tape with ordinary inches on top, diameter graduations underneath to a hundredth of an inch and metric on both sides.
This review is based on the maker’s specifications and on how diameter tapes are used in arboriculture and the trades generally. We have not used this tape ourselves.
How It Reads
Wrap the tape around the trunk at 4.5 feet, read where the diameter scale meets the zero and that is the diameter, assuming a round section, which is the forestry convention. The top side is a normal tape for everything else, and the metric scales on both faces save conversion. Owners in the trades use the same tape for pipe outside diameter and posts.
The Case
A magnet on the case sticks it to a truck or a steel post while you work, and the rotating wheel brake controls retraction with one hand, which matters when the other hand is holding the tape against bark. The reflective finish helps find it in leaf litter.
Range and Durability
Twelve feet of tape wraps a trunk about 45 inches across, which covers nearly every residential tree. The half-inch steel tape is narrow, so on deeply furrowed bark pull it snug and level, and keep it away from thorns; a kinked steel tape never retracts smoothly again. Foresters measuring old-growth use longer cloth tapes.
Who Should Buy It
- Arborists and landscapers recording diameter at breast height.
- Homeowners dosing tree fertiliser or injections by trunk inch.
- Anyone sizing pipe, posts or tree guards by outside diameter.
Common Owner Complaints
Narrow width on rough bark and the limit of twelve feet. Both are inherent to a pocket tape; it is not a forestry logger’s tape and does not claim to be.
Technique Tips
Keep the tape level around the trunk and pull it snug but not tight enough to compress loose bark. If the trunk forks below 4.5 feet, measure each stem and record both, which is the arborist convention. For a tree on a slope, measure from the uphill side. Note the date with each reading; annual diameter growth of a quarter to half an inch tells you a tree is healthy.
Alternatives to Consider
Lufkin and Spencer diameter tapes are the forestry standards at longer lengths. A fabric tailor’s tape and a calculator work for occasional measurements. Calipers suit small branches.
Verdict
If you measure trees, pipes or posts more than once a year, a diameter tape pays for itself in time and mistakes avoided, and this one adds a magnet, a brake and metric for a modest price. It belongs in every arborist’s pocket and most keen gardeners’ tool bags.
Full specifications
Perfect Pi Diameter and Circumference Tape Measure, 12 Foot, Imperial and Metric FAQs
The graduations are spaced pi times further apart than inches, so the circumference you wrap reads directly as diameter. It assumes a round trunk, which is the standard forestry convention.
At 4.5 feet above ground, the standard diameter at breast height, used for growth records, fertiliser rates, tree valuation and permit rules.
Tree fertiliser and injection doses are given per inch of trunk diameter, tree wrap and guards are sized by diameter, and many municipal tree ordinances set protection by diameter at breast height.
Yes. Outside diameter of pipe, posts and cylinders reads the same way.
