Fat Old Climber Mounjaro Calculator: What It Is and What to Know (October 2026)

01 · The tool

What the Fat Old Climber calculator is

It is a free, community-built calculator discussed in UK forums such as r/mounjarouk.

We do not run it, review its output or recommend it. Anything it tells you about partial, split or micro-doses falls outside the licensed use of Mounjaro.

02 · Safer route

Using the licensed doses instead

If a dose feels too strong, your prescriber can review it, which is safer than improvising with a calculator.

All six strengths are in stock, supplied through official LillyDirect from Eli Lilly's UK distribution. A free consultation is reviewed by a GPhC-registered prescriber, and ongoing monthly reviews cover dose changes and side-effect support.

03 · Cost

Planning cost without a calculator

Your first 2.5mg pen costs £129.99 with code FIRST40 (standard price £169.99). Every later order is £25 off the standard price. If you step up after 4 weeks on each dose, your first four pens (2.5mg to 10mg) cost £959.96 at standard prices. A first year costs £2,711.88 if you settle at 5mg, up to £3,599.88 at 15mg, before discounts.

Related guide: Mounjaro protein calculator.

04 · Why we avoid click counting

Why we avoid click counting

Click counting is not part of the licensed instructions. Our guide to Mounjaro clicks covers this in more detail.

Dose accuracy, pen life and storage all depend on following the pen's own instructions. A third-party tool cannot check any of those for you, and its output is only as good as the numbers typed in.

05 · Who Mounjaro suits

Who Mounjaro suits

Mounjaro is licensed for weight management in adults with a BMI of 30 or above, or 27 to 30 with a weight-related condition such as high blood pressure, prediabetes or type 2 diabetes. It is used alongside a reduced-calorie diet and increased physical activity.¹ A prescriber checks your BMI, history and current medicines first. If you are wondering about age, see how old you need to be.

In SURMOUNT-1 (2,539 adults), average weight loss over 72 weeks was 15% on 5mg, 19.5% on 10mg and 20.9% on 15mg, against 3.1% with lifestyle changes alone. For someone starting at 100 kg, 20.9% is about 21 kg. Trial averages do not predict your own result, and individual results vary.²

06 · If something went wrong

If you think you took the wrong amount

If you think you took too much Mounjaro, talk to a doctor immediately and tell them what you took and when. Report any suspected side effect through the Yellow Card scheme.¹⁹ Buy only from a GPhC-registered pharmacy; the MHRA publishes counterfeit medicine alerts.⁷ You can check us on the GPhC register (9012878).¹²

07 · Pens and old stock

Pens and leftover stock

Unopened pens stay in the fridge until first use and keep until the printed expiry date.¹ Once in use, a KwikPen can be kept out of the fridge at a temperature not above 30°C for up to 30 days after first use, and must then be thrown away. Do not use a pen that has been above 30°C or has been frozen.¹ For disposal, see what to do with an old Mounjaro pen. For a wider view of options, browse all weight loss treatments.

08 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹ Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly.