IV cannula sizes with gauge, ISO colour, OD and approximate flow rate in ml/min.
An intravenous (IV) cannula is a short, flexible catheter inserted over a needle into a peripheral vein, giving direct access to the bloodstream for fluids, medicines, blood products, contrast media, or blood sampling. Peripheral cannulation is one of the most frequently performed invasive procedures in healthcare — and choosing the right gauge for the clinical situation is the single decision that most affects whether the device works, how long it lasts, and how much it hurts the patient.
IV cannula sizes are expressed in gauge (G) on the Birmingham wire gauge scale, which runs counter-intuitively: the higher the gauge number, the narrower the cannula. A 14G is a large-bore device used for rapid resuscitation; a 24G is very fine and used for neonates, paediatrics, and fragile veins. Every gauge in routine clinical use also carries a standardised hub colour under ISO 10555-5, so the size can be identified at a glance without reading the packaging — a meaningful safety feature during an emergency, when a clinician can call for "a grey" or "a pink" and be understood immediately.
This reference covers gauges 14G through 26G with ISO colour codes, external diameter, catheter length, approximate gravity flow rates, and the clinical indications each size is suited to. It also covers the physics behind flow rate, evidence-based site selection, how to recognise the common complications early, and current guidance on when a cannula should be replaced — an area where practice has changed significantly and where a lot of published material online is now out of date.
A principle worth stating up front, because it runs against instinct: the correct choice is the smallest gauge that will deliver the prescribed therapy. Oversized cannulas occupy more of the vessel lumen, reduce blood flow around the catheter, and are associated with higher rates of mechanical phlebitis and thrombosis. Reaching for a large bore "just in case" is not a neutral decision — it carries a real cost to the patient and to the survival of the line.
Q \propto \frac{r^4}{L} \quad \text{(Hagen–Poiseuille)}
Inputs: Gauge selection by clinical scenario — ISO 10555-5 colour, approximate gravity flow rate, and indication
The 20G (pink) is the general-purpose default for stable adults, delivering roughly 54–60 mL/min — sufficient for the overwhelming majority of IV medicines and maintenance fluids while remaining comfortable to place in an average forearm vein. The 18G (green) is the standard choice where transfusion, CT contrast, or faster fluid replacement is anticipated. Between them, these two sizes account for most peripheral cannulations in adult practice. Reach for a larger bore only when the therapy genuinely requires the flow.
An 18G (green) is the accepted minimum for adult transfusion, with 16G preferred where rapid administration is required. The concern with narrower cannulas is twofold. Packed red cells are considerably more viscous than crystalloid, so transit through a fine lumen becomes impractically slow — a unit that should run over two to three hours may not complete within the four-hour limit for a unit at room temperature. Separately, forcing red cells through a narrow bore under pressure raises shear stress and the risk of haemolysis. In paediatrics, 22G or even 24G is routinely used for transfusion with a syringe pump or infusion device, because the volumes involved are proportionately smaller.
Current guidance favours replacement when clinically indicated rather than on a fixed routine schedule. The Infusion Nurses Society Standards of Practice (2021) and CDC guidance both support removing a peripheral cannula when it is no longer needed, or when there are signs of phlebitis, infiltration, extravasation, occlusion, leakage, or suspected infection — not simply because a set number of hours has elapsed. This changed after randomised evidence, notably the large Rickard et al. trial published in The Lancet, found no increase in phlebitis or bloodstream infection with clinically indicated replacement compared with routine change. Much of the material online still quotes the older 72–96 hour rule; check your own organisation's policy, as some still specify a maximum dwell time. Cannulas placed in an emergency without full aseptic technique remain an exception and should be resited within 24–48 hours.
Because flow through a tube scales with the fourth power of its internal radius, as described by the Hagen–Poiseuille relationship. Doubling the radius increases flow roughly sixteen-fold, so even a modest widening produces a dramatic gain. This is why an 18G delivers around twice the flow of a 20G despite the difference looking small in the hand, and why a 14G moves volume at a rate no amount of pressure will achieve through a 22G. It also explains a detail that is easy to overlook: flow is inversely proportional to length, so a shorter cannula of the same gauge will run faster than a longer one. In resuscitation, short and wide beats long and narrow every time.
The forearm is generally the preferred site in adults: it is comfortable, the veins are reasonably straight, and it avoids the flexion that shortens the life of a line. The dorsum of the hand is acceptable but tends to be more painful and more easily dislodged. Avoid areas of flexion such as the antecubital fossa for anything other than short-term or emergency use, since repeated bending kinks the catheter and promotes mechanical phlebitis. Avoid the side of a mastectomy with lymph node clearance, limbs with an arteriovenous fistula, areas of infection, burns, or planned surgery, and — where possible — the patient's dominant arm. Working distal to proximal preserves proximal veins for later attempts, which matters for anyone facing a long admission.
Both describe fluid leaking into the surrounding tissue rather than the vein, and the mechanism is identical — the distinction lies in what has leaked. Infiltration involves a non-vesicant solution and typically causes swelling, coolness, tightness, and slowed infusion; it is usually managed by stopping the infusion, removing the cannula, elevating the limb, and resiting. Extravasation involves a vesicant agent — many chemotherapy drugs, vasopressors, concentrated potassium, calcium salts, some contrast media — capable of causing tissue necrosis. Extravasation is a medical emergency: stop the infusion immediately, leave the cannula in place initially to allow aspiration of residual drug, and escalate per your organisation's extravasation protocol, which may include a specific antidote. The clinical stakes differ enormously even though the two look similar at first glance.
Phlebitis is inflammation of the vein wall and is the most common complication of peripheral cannulation. It presents as pain or tenderness at the site, erythema tracking along the vein, warmth, swelling, and in advanced cases a palpable venous cord or purulent discharge. Most services use a structured scale — the Visual Infusion Phlebitis (VIP) score is widely adopted — where 0 indicates a healthy site, 1 early signs warranting observation, 2 the beginning of phlebitis requiring resiting, 3 established phlebitis, and 4 to 5 advanced phlebitis or thrombophlebitis needing escalation and treatment. Documenting a score at each shift makes deterioration visible early. Mechanical causes (oversized cannula, siting over a joint, poor securement) are as common as chemical ones, which is why gauge selection and site choice matter so much.
Power injection of iodinated contrast for CT typically requires an 18G or 20G cannula sited in the antecubital fossa or forearm, because the injector delivers at high flow rates — commonly 3 to 5 mL/s. The cannula must be rated for power injection; not all peripheral catheters are, and using an unrated device risks catheter rupture or extravasation of contrast, which can cause significant tissue injury. Always confirm the local radiology protocol, as required gauge and site vary with the study and the injection rate prescribed.
Paediatric practice favours the smallest workable gauge, both because veins are smaller and because comfort and preservation of vascular access matter over a potentially long admission. In broad terms, 22G or 24G suits most infants and small children, 24G or 26G is used in neonates, and older children may tolerate 20G or 22G. Because flow through fine cannulas is limited, volumes are usually delivered by syringe driver or volumetric pump rather than gravity. Transfusion through 22G or 24G is routine in paediatrics with an appropriate device, in contrast to adult practice. Vein visualisation aids and topical anaesthetic preparation meaningfully improve first-attempt success in this group.
Peripheral access is unsuitable for several categories of therapy. Vesicant chemotherapy, long-term parenteral nutrition, highly concentrated or strongly hyperosmolar solutions, and prolonged courses of irritant medication all require central access — a PICC, tunnelled line, or port — because repeated exposure damages peripheral vessels and extravasation carries serious consequences. Vasopressor infusions are ideally given centrally, although short-term peripheral administration under close monitoring is accepted in some emergency settings. Anticipated treatment duration also matters: if IV therapy is likely to run beyond a week or two, referral for a longer-term device early spares the patient repeated resiting and preserves peripheral veins.
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