Tubing OD vs ID: Chemical Feed Sizing Reference | Apex Flow

The most common chemical-feed plumbing error is ordering tubing by the wrong diameter. Rigid tube and the compression or push-to-connect fittings that grip its outside are specified by outside diameter (OD); soft tubing and the barbs that go inside it are specified by inside diameter (ID). A "3/8 inch" compression tube and a "3/8 inch" barbed line are not the same part — one is 0.375" on the outside, the other is 0.375" on the bore. Mixing the two is why a fitting won't seal or won't go on at all. This guide gives the OD/ID/wall reference chart, the rule for which dimension a fitting uses, the flow and pressure factors, and a sizing troubleshooting table.

Apex Flow Solutions stocks chemical-feed tubing in PE, PVC, PTFE, PFA, and polypropylene across the common fractional and metric sizes. The dimensions below are nominal for standard-wall tube; always confirm OD, ID, and wall against the specific product and the pump or fitting it connects to.

Matching tubing to a metering pump or fitting?

Pump heads and fittings call out OD, ID, or both — and getting it wrong means a leak or no flow. Send us your pump model or fitting size, the chemical, and the back-pressure and our team will confirm the exact tubing size and wall.

OD vs ID: Why It Matters

A length of tubing has three dimensions: outside diameter, inside diameter (the bore), and wall thickness, where OD = ID + (2 × wall). Any two define the third. The reason the OD-versus-ID distinction trips people up is that different connection methods grip different surfaces. A compression or push-to-connect fitting seals on the outside of a rigid tube, so it must know the OD precisely — the ferrule or collet is machined to one OD. A barbed fitting is inserted inside soft tubing, so it is sized to the ID. Order tubing by the dimension your connection method uses, and always state which one you mean. "1/2-inch tubing" is ambiguous until you say OD or ID.

Which Dimension Does Your Fitting Use?

Match the sizing convention to the connection type before you order tube.

Connection method	Sized by	Tube type	Critical tolerance
Compression (ferrule)	OD	Rigid/semi-rigid	OD held tight; round, undamaged
Push-to-connect (collet)	OD	Rigid/semi-rigid	OD + square-cut end
Hose barb	ID	Soft/flexible	ID matches barb nominal
Flare (37°/45°)	OD	Rigid metal/some plastic	OD + flareable wall
Metering pump head	OD & ID	Per pump spec	Both must match the head fitting

Note that metering pump connections often call out both OD and ID because the tube slides over a barb (ID) and is captured by a coupling nut on the OD — the classic "4/6" or "1/4 × 3/8" tubing callout means 1/4" ID by 3/8" OD.

Cross-section of tubing labeling outside diameter, inside diameter, and wall thickness with a compression fitting on the OD and a barb on the ID

OD = ID + twice the wall. Compression and push-to-connect fittings grip the OD; barbs insert into the ID. Order by whichever dimension your connection method uses.

OD / ID / Wall Reference Chart

Common chemical-feed tubing sizes. The "OD × ID" callout column is how dual-dimension tubing is ordered; wall is the resulting nominal thickness.

Common callout (OD × ID)	OD (in)	ID (in)	Wall (in)	Metric equiv (OD×ID mm)
1/4" × 1/8"	0.250	0.125	0.063	6.4 × 3.2
1/4" × 0.170"	0.250	0.170	0.040	6.4 × 4.3
3/8" × 1/4"	0.375	0.250	0.063	9.5 × 6.4
1/2" × 3/8"	0.500	0.375	0.063	12.7 × 9.5
5/8" × 1/2"	0.625	0.500	0.063	15.9 × 12.7
3/4" × 5/8"	0.750	0.625	0.063	19.1 × 15.9
1" × 3/4"	1.000	0.750	0.125	25.4 × 19.1

ID, Flow & Velocity

Flow capacity is governed by the ID, not the OD, and it scales with the square of the ID because cross-sectional area is π/4 × ID². Doubling the ID quadruples the area at a given velocity. For chemical feed, the practical guidance is to keep liquid velocity in the rough range of 2–5 ft/s on suction and up to about 5–7 ft/s on discharge: too slow and solids settle or air pockets stall the line, too fast and you get excess friction loss, water hammer, and noise. As a rule of thumb, a 1/4" ID line suits low-flow metering (fractions of a GPM), 3/8"–1/2" ID covers most small chemical-feed duties, and 3/4"+ ID is for transfer rather than dosing. Size the ID for the flow and velocity; the OD then follows from the wall you need for pressure.

Wall Thickness & Pressure Rating

For a given material, pressure rating rises with wall thickness and falls with ID — the governing relationship is the dimension ratio (OD ÷ wall). A thicker wall at the same OD means a smaller bore but a higher pressure rating. That is why two tubes with the same 3/8" OD can have very different ratings: a 1/4" ID (0.063" wall) tube holds far more pressure than a thin-wall 0.305" ID version. Pressure rating also drops sharply with temperature — most thermoplastic tubing is rated at 73°F and may lose half its rating or more by 120°F. Never size tubing on bore alone; check the working-pressure rating at your actual operating temperature, and apply the manufacturer's de-rating factor.

Sizing for Metering Pumps

Chemical metering pumps are the most common reason to get tubing sizing exactly right. The pump head's suction and discharge fittings are specified by the manufacturer as an OD × ID pair (for example 1/4" × 3/8" or 0.5" × 0.375"), and the tube must match both — the ID slides over the barb and the OD is captured by the coupling nut. Undersized ID on the suction side starves the pump and causes loss of prime; oversized tubing on the discharge can mask back-pressure problems and worsen siphoning. The discharge tubing must also be rated above the system back-pressure with margin. When a pump specifies a back-pressure or anti-siphon valve, the tubing run downstream must hold that pressure continuously. Always match the tubing to the pump's published fitting size first, then verify the pressure rating against the discharge head.

Metering pump head showing suction and discharge tubing fittings labeled with OD by ID callouts

A metering pump head calls out tubing as an OD × ID pair (e.g. 1/4" × 3/8"): the ID slides over the barb and the OD is captured by the coupling nut. Both dimensions must match the pump spec.

Sizing Troubleshooting

Most tubing problems are an OD/ID mix-up, a wall/pressure mismatch, or a flow-velocity error. Diagnose with this table.

Problem	Likely cause	Fix
Compression fitting won't seal	Tube OD doesn't match fitting size	Order tube by OD to match the ferrule
Barb too loose / blows off	Tube ID larger than barb nominal	Match ID to barb; secure with clamp
Tubing balloons or bursts	Wall too thin / over temp-rating	Use thicker wall; de-rate for temperature
Pump loses prime	Suction ID undersized / restricted	Match suction ID to pump spec; shorten run
Air pockets / solids settle	Velocity too low (ID too big)	Reduce ID to raise velocity to 2–5 ft/s
Water hammer / noise	Velocity too high (ID too small)	Increase ID; add pulsation damping

Standards & References

Plastic tubing dimensions and pressure ratings reference ASTM D1785 / D2241 (PVC pipe/tube), the relevant PE and fluoropolymer tubing specs, and the manufacturer's published OD/ID tolerance and working-pressure rating at temperature. Compression fitting tube ODs follow the fitting standard (e.g. SAE J512 / instrumentation tube-fitting practice). Flare tube sizing follows SAE J533. For potable or food-contact chemical feed, confirm the tubing carries NSF/ANSI 51 or 61 certification as applicable. Always verify the pressure rating at your operating temperature, not the 73°F catalog figure.

Frequently Asked Questions

Is tubing measured by OD or ID?

It depends on the connection. Compression and push-to-connect fittings grip the OD, so that tube is sized by OD. Barbed fittings go inside the tube, so that tube is sized by ID. Always state which dimension you mean.

What does "1/4 × 3/8" tubing mean?

It is a dual-dimension callout: 1/4" inside diameter by 3/8" outside diameter, giving a 1/16" wall. Metering pumps commonly specify tubing this way because the tube fits over a barb (ID) and into a coupling nut (OD).

How do I pick the ID for my flow rate?

Size the ID so liquid velocity lands around 2–5 ft/s. Too low and solids settle or air stalls the line; too high and you get friction loss and water hammer. Flow area scales with the square of the ID.

Why did my tubing burst below its rating?

Almost always temperature. Catalog ratings are at 73°F and can drop by half or more at 120°F. De-rate the working pressure for your actual operating temperature and choose a thicker wall if needed.

Does a bigger OD mean higher pressure rating?

Not by itself. Pressure rating depends on the OD-to-wall ratio. At the same OD, a thicker wall (smaller bore) holds more pressure. Check the rating, not just the dimensions.

My metering pump keeps losing prime — is it the tubing?

Often, yes. An undersized or overly long suction line, or one with a smaller ID than the pump spec, starves the head. Match the suction ID to the pump's fitting size and keep the run short and rising.