Horizontal directional drilling

HDD reamer size calculator

Ream at least 1.5 times the widest thing you are pulling. The rule fits in a sentence; the work is in the word “widest” — the pipe’s real outside diameter rather than its nominal size, the couplings and the pulling head, and for a bundle of conduits a diameter set by how they lie together. The calculator below works all of it out, applies the 12 in clearance cap for large pipe, and picks from the backreamers you actually have.

Size the reamer

Give the number of conduits and the outside diameter of one; a single pipe is a bundle of one. If one pipe in the bundle is larger than the rest, enter it as the carrier. The calculator opens on a single 4 in IPS pipe, which is 4.500 in across. Every figure the answer depends on is in the table under it.

What you are pulling

Units Number of conduits, not counting a carrier (1 to 30) Outside diameter of each conduit (in) Or fill it from an IPS size

Optional: carrier, joints, pulling head, a measurement

Carrier pipe outside diameter (in); blank if every conduit is the same size Coupling or bell outside diameter (in), if wider than the pipe Pulling head or grip outside diameter (in) Measured bundle diameter (in); replaces the calculated one Loose-bundle allowance for 4 or more identical conduits (%)

Sizing rule

Hole ÷ widest point (PPI, Vermeer, Melfred Borzall: 1.5) Over 24 in (609.6 mm), no more than 12 in (304.8 mm) of clearance on the diameter (PPI; HDD Consortium) Use 1.3 from 10 in (254 mm) product up (Vermeer fluid guide)

Your reamers and the bore

Reamer sizes you have (in), separated by commas. The example is every even size from 4 to 48 in; replace it with yours. Bore length (ft), for the total hole volume

What it assumes. The widest point, W, is the smallest circle around everything at the fattest cross-section of the string. For identical conduits the bundle is the tightest packing known, which is proven the tightest possible for 1 to 14 conduits and for 19; a made-up bundle usually lies looser, and that is what the allowance, applied from 4 conduits up, is for; its 10 per cent default comes from the one measured bundle found (see the worked examples), not from a rule. With a carrier, the smaller ducts are placed in one row against it, touching each other: an arrangement you can build, not one proven the tightest. Spread around the carrier they make a wider bundle, up to the carrier plus two ducts, so measure it if they will be banded that way. A coupling diameter is applied to every conduit at once, as if all the joints lined up; with a carrier, it is taken as the carrier’s. The 12 in cap is on the diameter — hole minus W, so 6 in all round — and, as PPI states it, applies only when W is over 24 in. When the cap governs, the reamer picked is the largest on your list that stays under it; otherwise it is the smallest that meets the ratio. Hole volume is a clean cylinder the diameter of the reamer.

The rule, and what it is measured on

Tooling makers and the Plastics Pipe Institute give the same number. Vermeer’s advice on choosing a reamer is at least 1.5 times the widest product pipe pack being pulled through. Melfred Borzall’s rule of thumb is a backreamer 1.5 times the outside diameter of the pipe, taken at its widest point: where two sections slip together the joint can be up to 2 in wider than the pipe. PPI’s guidance for mini-HDD recommends a finished hole at least 50 per cent larger than the outside diameter of the pipe or pipe bundle, and says the enlargement has to pass the pulling grips as well as the pipe.

PPI adds a ceiling: no more than 12 in of clearance for pipes or bundles over 24 in. Lawrence Slavin’s paper on pulling tensions, among PPI’s Municipal Advisory Board publications, attributes that maximum to the HDD Consortium’s Good Practices Guidelines (2008), and defines clearance as the borehole diameter minus the pipe’s outside diameter. The cap is on the diameter, then — 6 in all round. Together:

W = max( bundle diameter, coupling or bell, pulling head or grip ) D_hole = min( 1.5 · W, W + 12 in ) W = the widest point anywhere on the product string · D_hole = the diameter to ream · 12 in (304.8 mm) = the largest recommended clearance, D_hole − W, which PPI states for W over 24 in. The calculator applies it only there, whatever ratio you set; with 1.5 it could not bind anywhere else.

The two limits meet at exactly 24 in, where 1.5 × 24 and 24 + 12 are both 36. Below 24 in the 50 per cent rule governs; above it the cap does, so the ratio of hole to pipe falls as the pipe grows: 42 in for a 30 in pipe (1.4 times), 60 in for a 48 in pipe (1.25 times).

Slavin gives the reasons for both ends. The 50 per cent minimum keeps drilling fluid circulating and carries the spoils out. A bigger hole eases the pipe through and reduces the effect of its stiffness in the bends, but makes reaming and spoils removal harder, and he leaves the optimum to engineering judgement. He also finds that the exact hole diameter has only a small effect on the pull load: the fluid-drag term on the pullback force page grows with the hole, and stays small.

Other conventions you will meet

PPI’s Handbook of PE Pipe gives normal oversizing as 1.2 to 1.5 times the carrier pipe, depending on soil type and stability, depth, drilling mud and borehole pressure.
Vermeer’s drilling-fluid guide (2019) takes the reamer as 1.5 times the product up to 9 in and 1.3 times from 10 in up. It does not say which applies between 9 and 10 in; the calculator’s option keeps 1.5 until 10 in.
Melfred Borzall lists where less than 1.5 can do — solid rock, on-grade bores such as sewers that need a tight hole, and short bores under 150 ft where the cuttings clear easily — and where the full 1.5 is needed: cobble, sand and other unstable ground, and clay, which swells with drilling fluid and shrinks back as it dries, and may want a little more.
Vermeer adds that bore length, ground conditions and the bend the product can take may call for opening the hole beyond 1.5 times.

Nominal size is not the diameter

Vermeer’s worked example is a 4 in product needing at least a 6 in reamer. That is exact for something 4.000 in across. A 4 in IPS polyethylene pipe or conduit is not: it is 4.500 in outside, in WL Plastics’ IPS pipe chart (sizes per ASTM D3035 and F714) and in Southwire’s SDR 11 conduit sheet (ASTM F2160) alike. The same rule on the real diameter asks for 6.75 in, and a 6 in reamer is only 1.33 times that pipe. Every IPS size below 14 in is larger than its name; from 14 in up the name is the outside diameter.

The hole to ream for IPS pipe, by the rule above with the 12 in cap, before any coupling or pulling head:

IPS size	Outside diameter	1 pipe	2 pipes	3 pipes	4 pipes, square
1 in	1.315 in	1.97	3.95	4.25	4.76
1¼ in	1.660 in	2.49	4.98	5.37	6.01
1½ in	1.900 in	2.85	5.70	6.14	6.88
2 in	2.375 in	3.56	7.13	7.68	8.60
2½ in	2.875 in	4.31	8.63	9.29	10.41
3 in	3.500 in	5.25	10.50	11.31	12.67
4 in	4.500 in	6.75	13.50	14.54	16.30
6 in	6.625 in	9.94	19.88	21.41	23.99
8 in	8.625 in	12.94	25.88	27.88	31.23
10 in	10.750 in	16.13	32.25	34.74	37.95
12 in	12.750 in	19.13	37.50	39.47	42.78
14 in	14.000 in	21.00	40.00	42.17	45.80
16 in	16.000 in	24.00	44.00	46.48	50.63
18 in	18.000 in	27.00	48.00	50.78	55.46
20 in	20.000 in	30.00	52.00	55.09	60.28
22 in	22.000 in	33.00	56.00	59.40	65.11
24 in	24.000 in	36.00	60.00	63.71	69.94

Hole diameters in inches. Two or three conduits banded together have only one compact arrangement, so those columns are exact; four can sit square or diamond, and a diamond is 13 per cent wider (see the next section). The four-pipe column is the square with no loose-bundle allowance; the calculator adds its allowance on top. Add couplings and the pulling head before using any of these, and round up to a reamer you have.

Measure the reamer, not its label

Melfred Borzall’s method: lay a straightedge level across the top cutter, parallel to the shaft, measure from the middle of the shaft up to it, and double the figure. Measure to the furthest edge of the widest cutter, not the body — measuring the body understates the tool and the hole comes out much bigger than planned. The stamped size is only a starting point; broken or worn cutters change what a reamer actually cuts.

How wide a bundle of conduits is

A bundle’s widest point is the smallest circle that holds all of its conduits — the circle the reamer has to clear. For equal conduits that is a classical problem in geometry, packing equal circles in a circle, and for small counts it has exact answers. As multiples of one conduit’s outside diameter, d:

2 conduits 2d 3 conduits (1 + 2/√3) d = 2.1547 d 4 conduits (1 + √2) d = 2.4142 d 5 conduits (1 + √(2(1 + 1/√5))) d = 2.7013 d 6 or 7 3d Proven the tightest possible for 1 to 14 conduits and for 19; for the other counts these are the best packings anyone has found. Closed forms and proofs as tabulated in Circle packing in a circle (Wikipedia). The calculator uses E. Specht’s Packomania table for 1 to 30, which agrees with every closed form here to ten decimal places.

Conduits	Bundle ÷ d	Conduits	Bundle ÷ d	Conduits	Bundle ÷ d
1	1.0000	11	3.9238	21*	5.2523
2	2.0000	12	4.0296	22*	5.4397
3	2.1547	13	4.2361	23*	5.5452
4	2.4142	14	4.3284	24*	5.6517
5	2.7013	15*	4.5214	25*	5.7528
6	3.0000	16*	4.6154	26*	5.8282
7	3.0000	17*	4.7920	27*	5.9064
8	3.3048	18*	4.8637	28*	6.0149
9	3.6131	19	4.8637	29*	6.1386
10	3.8130	20*	5.1223	30*	6.1977

* Best packing known, not proven. Every other count is proven the tightest possible. Source: Packomania; proofs as listed by Wikipedia.

Six and seven give the same answer because a seventh conduit fits in the middle of a ring of six, and nineteen fits in the same circle as eighteen. Some counts come free.

An area formula gets small bundles wrong

A shortcut treats the bundle as the total conduit area divided by a fill factor φ, and takes the diameter of a circle with that area: D = d √(n/φ). It cannot fit every count, because the true ratio does not grow as √n. With φ = 0.7 it makes two conduits 1.69d wide — 15 per cent narrower than two pipes side by side can physically be — and twenty-four conduits 3.6 per cent wider than the best packing known. Undersize the bundle and the reamer is undersized by the same fraction.

Real bundles lie looser

The figures above are the tightest arrangements there are, or for the starred counts the tightest known. Banded tight, two conduits and three conduits can only settle one way, and it is that one. From four up they need not. The length of a band around a bundle is the perimeter of the polygon through the conduit centres plus πd, and for four conduits the square and the diamond have the same polygon, four sides of length d. The band cannot tell them apart, but the diamond is (1 + √3)d = 2.732d across, 13 per cent wider than the square.

The only measured figure found for this page is the 24-conduit bundle in the worked examples below, which came out 10 per cent wider than the tightest arrangement known. The calculator’s loose-bundle allowance defaults to that 10 per cent for four or more identical conduits. It is one job, not a rule. Where you can, measure the made-up bundle, pulling head and all, and enter the measurement.

Checked against the published figures

A 4 in product (Vermeer)

Enter one conduit of 4 in and the calculator returns 6.00 in, Vermeer’s 6 in reamer. Enter the 4 in IPS pipe’s real 4.500 in and it returns 6.75 in; from the example list it picks 8 in, because 6 in falls short.

A 24 in pipe (PPI)

Slavin’s reference case for pulling-tension estimates is a 24 in DR 11 HDPE pipe with a hole-to-pipe ratio of 1.5: a 36 in hole and 12 in of clearance, listed in his tables as 50 per cent clearance. The calculator returns 36.00 in with the cap on or off, because 24 in is where the two limits cross. For that hole the same paper tabulates the fluid-drag increment at 10 psi as 2,827 lb, which is 10 × (π/8) × (36² − 24²).

Twenty-four 4 in conduits in one pull

Trenchless Technology (December 2022) reported a PPI Project of the Year in which Vision Directional Drilling pulled 24 runs of 4 in SDR 11 HDPE conduit as one piece, in 500 to 600 ft segments. The article gives the bundle as 28 in in diameter and the reamer as 36 in.

The article gives only the nominal size. Taking the conduit as IPS, 4.500 in outside, the tightest arrangement of 24 known is 5.6517 × 4.5 = 25.43 in, so the bundle as made up was 10 per cent wider. On the 28 in bundle the rule gives 40 in: 1.5 × 28 is 42, which is over the 12 in cap, so 28 + 12. The crew used 36 in — 1.29 times the bundle and 4 in under the guideline — and the article reports pulls often of 18,000 lb or less. Enter 24 conduits of 4.5 in with a measured bundle of 28 in and the calculator returns the 40 in. Leave the measurement out and its default 10 per cent allowance gives 27.98 in; that agreement is not a check, because the allowance was taken from this job.

How much hole that is

Vermeer’s fluid guide works an example of a 4 in gas line behind a 6 in reamer and takes that hole as 1.47 gal per foot. A 6 in cylinder is (π/4) × 6² × 12 in ÷ 231 in³ per gallon = 1.469 gal/ft, which is what the calculator reports for a 6 in reamer. How much fluid to pump into that hole depends on the soil; that is the drilling fluid calculator.

What the rule leaves out

Getting to size. PPI’s guidance is that reaming and pulling in a single pass suits product up to about 4 in nominal, and that larger holes should be pre-reamed in stages. Vermeer’s example is that an 8 in hole cannot be taken to 30 in in one go.
Too big is not free. Melfred Borzall lists the costs: bigger tooling, more drilling fluid, slower progress, the risk of encroaching on other lines, and pipe that can sink with time.
Too small is worse. Melfred Borzall again: the job drags, the road can heave during pullback, and the product can stretch or break.
The hole is not the reamer. Everything here assumes a clean hole the size of the reamer. Nothing on this page predicts washout, squeeze or collapse; the ground and the drilling fluid set those.
Clearance to other utilities. The hole wall sits half the reamer diameter from the pilot centreline, and that comes out of every crossing clearance. Potholing and clearance after reaming works it through.
Bending. The string has to follow the bore’s curves as well as fit the hole. The limits on curvature are on the minimum bend radius page.

Carrying it into the plan

BorePlanner Coming soon, our bore planner for Android and Windows, checks every utility crossing against the wall of the reamed hole rather than the pilot rod. It takes that hole as 1.5 times the product diameter you enter, and it has no separate reamer or bundle field. For a bundle, enter the widest point from this page as the product diameter. If you will pull a reamer larger than 1.5 times that, BorePlanner’s clearances are optimistic by the difference in radius; entering the reamer diameter divided by 1.5 makes them exact.

Related: laying out the bore path profile, estimating pullback force, and the rest of the free tools.

All our instruments →

Sources

Vermeer, “Top five questions you should ask when selecting the right reamer” (28 September 2021) — at least 1.5 times the widest product pipe pack, the 4 in to 6 in example, when to open beyond 1.5, pre-reaming in stages.
Vermeer, “Making sense of HDD drilling fluids, Part 2” (10 May 2019) — 1.5 times up to 9 in, 1.3 times from 10 in; 1.47 gal/ft for a 6 in reamer.
Melfred Borzall, “How to Properly Measure Your Reamer and Choose the Right Size for the Job” (December 2017) — 1.5 times the outside diameter at the widest point, joints up to 2 in wider, ground-condition exceptions, how to measure a reamer.
Plastics Pipe Institute, Municipal Advisory Board, “Back-Ream” — hole at least 50 per cent over the pipe or bundle, 12 in maximum clearance over 24 in, pulling grips, pre-reaming above about 4 in.
L. M. Slavin, Horizontal Directional Drilling: Estimating Pipe Pulling Tensions (PPI Municipal Advisory Board publications; revision dated 3 September 2026) — definition of clearance, the 50 per cent minimum and its purpose, the 12 in maximum, the 24 in reference case and its 2,827 lb fluid-drag increment.
Plastics Pipe Institute, Handbook of PE Pipe, Chapter 12 — normal oversizing of 1.2 to 1.5 times.
WL Plastics, WL102 IPS Pipe Sizes & Pressure Class — IPS outside diameters, sizes per ASTM D3035 and F714.
Southwire, spec 87003, EPEC 11 (SDR 11) HDPE conduit — conduit outside diameters per ASTM F2160, 3/4 to 4 in, including 2½ in at 2.875 in.
Trenchless Technology, “Record-number of Conduit Runs Pulled as Single Shot Using HDD” (12 December 2022) — 24 runs of 4 in SDR 11 conduit, 28 in bundle, 36 in reamer, pull loads.
E. Specht, Packomania, “The best known packings of equal circles in a circle” (table updated 25 December 2024) — the ratios and circle centres used for 1 to 30 conduits.
“Circle packing in a circle”, Wikipedia — closed forms, and which counts are proven optimal.
HDD Consortium, Horizontal Directional Drilling Good Practices Guidelines, 3rd edition (2008) — as cited by the two PPI documents above; not consulted directly.