IC, EC, FRC, VC, TLC: which volumes go into which capacity? My worked answer doesn't match the key
Ananya Reddy11th Sample
27 Sept 2026 · 1 min read
Colour-coded my respiratory volumes and capacities page (TV in green, IRV in yellow, ERV in pink, RV in grey) and I still failed a small numerical.
The question gave: tidal volume 500 mL, inspiratory reserve volume 3000 mL, expiratory reserve volume 1100 mL, residual volume 1200 mL. It asked for IC, EC, FRC, VC, TLC and pulmonary ventilation if the breathing rate is 12 per minute.
What I wrote:
- IC = TV + IRV = 3500 mL (fine)
- EC = TV + ERV = 1600 mL (fine)
- FRC = TV + RV = 1700 mL (marked wrong)
- VC = IRV + ERV = 4100 mL (marked wrong)
- TLC = ... I wasn't sure.
My notes say "FRC is the volume left in the lungs after a normal expiration", and "VC is the maximum volume you can breathe out after a forced inspiration". If I know the definitions, why do I put the wrong volumes together? Is there a way to derive the formulas from the definitions instead of memorising them?
Also, how do I calculate pulmonary ventilation? I've seen mL/min mentioned. Is that a fixed value or does it depend on the question?
7 answers & replies
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[+][–]Meera Patel11thSample· 22d ago
You can derive them. Draw the volumes as a stack from the top, and think of the lung at three points: the full-breath point, the end of a normal breath-out, and the end of a forced breath-out.
- After a normal expiration, what stays in the lungs? The air that you could still force out (ERV) plus the air you can never force out (RV). So FRC = ERV + RV. Not TV, since TV has already left.
- Maximum breath out after a maximum breath in: you take the whole breath range. So VC = IRV + TV + ERV.
- Everything the lung can hold: TLC = VC + RV = IRV + TV + ERV + RV.
- IC = TV + IRV (from the end of a normal expiration to the maximum), EC = TV + ERV (from the end of a normal inspiration to the lowest).
[+][–]Ananya Reddy11thSample· 22d agoAuthor
the "what stays in the lung after a normal breath out" thing, that's the derivation I was missing. FRC = ERV + RV. Fixing all my cards right now (in the correct colour this time) 🎨
[+][–]Nisha Verma12thSample· 22d ago
Your numbers with the right formulas:
FRC = 1100 + 1200 = 2300 mL VC = 3000 + 500 + 1100 = 4600 mL TLC = 4600 + 1200 = 5800 mL Pulmonary ventilation (minute volume) = breathing rate x tidal volume = 12 x 500 = 6000 mL/min.
It isn't fixed. NCERT says a healthy human breathes in and out around 6000 to 8000 mL of air per minute, but the question gives you the breathing rate and tidal volume, so use those. Rate or TV go up during exercise, so the value goes up.
[+][–]Lakshmi TripathiDropperSample· 22d ago
I think VC should be IRV + ERV because vital capacity is the extra air you can move beyond your normal breathing, so tidal volume should not be included.
[+][–]Siddharth RaoDropperSample· 22d ago
Lakshmi, a lot of people think like this but vital capacity includes tidal volume. VC is the maximum air you can breathe out after a maximum breathing in. During that, you move the tidal air also, along with the reserve volumes on both sides. If you leave TV out, you're giving the reserve volumes only.
[+][–]Tanvi Deshmukh12thSample· 22d ago
Trick!! RV is the sneaky one, the air that never leaves. Only two capacities contain it: FRC and TLC. IC, EC and VC don't contain RV (a spirometer can't measure RV for the same reason, it can't take it out of the lungs).
So when a question asks you to compute VC, TLC − RV is another route to the same number: . Always cross-check by this route in the exam, it takes 5 seconds!!
[+][–]Farhan ShaikhDropperSample· 22d ago
Also remember typical values from NCERT: TV about 500 mL, IRV 2500–3000 mL, ERV 1000–1100 mL, RV 1100–1200 mL. If a question gives you numbers far outside that, it's probably a made-up number, so just use the formulas.
