Finding the deltoid injection site

Key points

• 01What the deltoid site is. The target is the thick central part of the deltoid muscle on the outer upper arm, a few finger widths below the bony shelf at the top of the shoulder. It is the site used for most adult vaccinations and for small-volume intramuscular medications.
• 02The acromion is the only landmark that matters. Every version of the instruction starts at the lower edge of the acromion process. If you cannot feel that bony ridge, you cannot place the site, and estimating from the shoulder seam of a shirt is not a substitute. Feel for bone first.
• 03Two to three finger widths, using the right fingers. The common teaching is two to three finger widths below the acromion. When you inject someone else, the measurement is taken with their proportions in mind, since a large hand on a small arm pushes the site down toward the narrow part of the muscle.
• 04The too-high error is the one with a name. Injecting above the target, into the bursa under the acromion, is the mechanism described in reports of shoulder injury related to vaccine administration. A 2025 paper in Vaccine looked at the incidence across three years of COVID-19 vaccine administration.
• 05The too-low error has its own risk. Below the level of the armpit the muscle thins and the radial nerve runs closer to the surface. The site is bounded on both ends for a reason, and the armpit check is the fastest way to confirm you have not drifted down.
• 06Volume is the real limit on this site. Nursing references treat the deltoid as a small-volume site and send larger volumes to the hip. If your prescriber has moved you from the arm to the ventrogluteal site, the volume of your medication is the usual reason.
• 07Needle length is a fat-pad question. A 2023 systematic review in BMJ Open asked which variables should inform needle length for deltoid intramuscular injection. A 2022 paper in Scientific Reports estimated deltoid subcutaneous fat pad thickness for the same reason. The right length is individual.
• 08Too short and the medication lands in fat. A 2022 study in Vaccine looked at needle length and skin-to-deltoid-muscle distance in adults receiving an mRNA vaccine. A needle that stops short of the muscle turns an intramuscular dose into a subcutaneous one, which is a different absorption profile.
• 09Relaxing the arm is part of the technique. A braced or raised arm tightens the muscle, which the nursing literature associates with more pain. Sitting with the arm hanging and the elbow slightly bent takes a second and changes the experience more than most people expect.
• 10The upper arm is also a subcutaneous site, and that is a different spot. The subcutaneous upper-arm site is the loose fatty area at the back of the arm, pinched up between the shoulder and the elbow. The deltoid is the muscle on the outer side. Same limb, different tissue, different needle.
• 11Alternate arms and write it down. On a repeated course, alternate sides and record which arm you used. Repeated injection into the same patch of muscle is associated with hardened tissue and unreliable absorption. Our injection record guide has a layout with a side column.
• 12Get signed off before you do it alone. Landmarking your own shoulder in a mirror is harder than it looks, and the safety margin at this site is measured in centimetres. Ask the nurse who trains you to watch you find the acromion and place the spot before you inject unsupervised.

Pre-injection checklist

• ✓The deltoid is an intramuscular site. GLP-1 medications, insulin and low molecular weight heparin are subcutaneous and do not belong in muscle. Our post on intramuscular versus subcutaneous injection explains why the route changes the medication.
• ✓Check the volume. Nursing references treat the deltoid as a small-volume site, and anything larger is usually redirected to the hip. The number that governs yours is on your label and from your prescriber.
• ✓Bare the whole shoulder. Rolling a sleeve up to a tight band pushes the arm tissue around and hides the landmark, which is how the too-high error happens.
• ✓Needle length for the arm depends on the fat pad over the muscle, which varies a lot between people. The length is prescribed for you rather than picked from a chart.
• ✓This is a technique reference, not medical advice. Your medication, route, dose, site and needle are set by your prescriber.

Step-by-step guide

1. 1

   Wash your hands and set up

   Wash with soap and water for at least 20 seconds and dry on a clean towel. Lay the syringe, needle, alcohol pads and gauze on a clean surface, and open the sharps container before you start so the used needle has somewhere to go immediately.

   Pro tip

   Always check the fluid is clear and colourless before injection.
2. 2

   Draw up the dose

   Draw the volume your prescriber set, following the routine in our guide to drawing up medication from a vial. If your instructions say to change to a fresh needle before injecting, do it now, because a needle that has been through a rubber stopper is duller than a new one.

   Pro tip

   Always check the fluid is clear and colourless before injection.
3. 3

   Expose the whole shoulder and let the arm hang

   Take the arm out of the sleeve rather than rolling the fabric into a band above the site. Sit with the arm relaxed at the side, elbow slightly bent. A flexed, braced arm tightens the deltoid and makes both the landmark and the injection harder.

   Pro tip

   Always check the fluid is clear and colourless before injection.
4. 4

   Find the acromion process

   Run your fingers along the top of the shoulder until you feel the bony ridge at the outer end, where the collarbone meets the shoulder blade. That hard shelf is the acromion process, and every measurement for this site is taken from its lower edge.

   Pro tip

   Always check the fluid is clear and colourless before injection.
5. 5

   Measure down from the lower edge of the acromion

   Nursing references place the target roughly two to three finger widths below the lower edge of the acromion, in the thickest part of the muscle. Use the recipient's finger widths, not yours, when injecting someone else, since hand size varies.

   Pro tip

   Always check the fluid is clear and colourless before injection.
6. 6

   Check the lower boundary against the armpit

   The target sits above the level of the armpit. The muscle narrows below that, and the radial nerve and the humerus come closer to the surface. If your chosen spot is level with or below the armpit fold, move up.

   Pro tip

   Always check the fluid is clear and colourless before injection.
7. 7

   Picture the inverted triangle

   The classic teaching draws a triangle with its base along the acromion and its point down at the level of the armpit, then injects in the middle of that triangle. It is the same target described a second way, which is useful as a cross-check.

   Pro tip

   Always check the fluid is clear and colourless before injection.
8. 8

   Clean the skin and let it dry

   Wipe the spot with an alcohol pad in one direction and let it air-dry for at least ten to fifteen seconds. Do not fan it or blow on it. Injecting through wet alcohol is the most common avoidable cause of stinging at any site.

   Pro tip

   Always check the fluid is clear and colourless before injection.
9. 9

   Insert at 90 degrees and inject slowly

   Hold the syringe like a dart, steady the skin, and insert at a right angle to the skin surface in one smooth movement. Follow your local instruction on aspiration, then push the plunger slowly and evenly. A fast push raises tissue pressure and is associated with more pain.

   Pro tip

   Always check the fluid is clear and colourless before injection.
10. 10

    Withdraw, press and dispose

    Pull the needle straight out along the line it went in, then press a gauze square on the spot without rubbing. Put the whole syringe with the needle attached into the sharps container without recapping. Write down the date, the medication and which arm you used.

    Pro tip

    Always check the fluid is clear and colourless before injection.

After the injection

• Press a gauze square on the site for a few seconds without rubbing.
• Move the arm normally afterwards. Gentle use is generally more comfortable than holding the shoulder still.
• Record the date, the medication and which arm you used, so the next injection goes on the other side.
• Put the syringe in the sharps container with the needle attached, and never recap it.

Common mistakes

Frequently asked questions

Sources

1. What variables should inform needle length choice for deltoid intramuscular injection? A systematic review. BMJ Open, 2023.
2. Statistical estimation of deltoid subcutaneous fat pad thickness: implications for needle length for vaccination. Scientific Reports, 2022.
3. The effect of needle length and skin to deltoid muscle distance in adults receiving an mRNA COVID-19 vaccine. Vaccine, 2022.
4. What three years of COVID-19 vaccine administration reveals about the incidence of shoulder injury related to vaccine administration. Vaccine, 2025.
5. Ogston-Tuck S. 2014. Intramuscular injection technique: an evidence-based approach. Nursing Standard 29(4):52-59.