A lot size is the fixed number of units in one derivatives contract. You cannot buy 10 NIFTY options — you buy one lot, and the lot contains however many units the exchange currently specifies. Every order must be a whole multiple of it.
What a lot size actually is
Exchange-traded derivatives are standardised. Rather than letting every participant trade an arbitrary quantity, the exchange fixes a contract size so that every NIFTY option contract is identical to every other one. That standardisation is what makes the contracts fungible and the order book liquid.
The practical consequence is that your minimum position is one lot, and your position sizing moves in steps of one lot rather than continuously. For a small account trading index options, this is often the binding constraint on strategy design — not your view on the market.
How to work out what one lot costs
For an option you are buying
The premium is quoted per unit. What leaves your account is the premium multiplied by the lot size.
| Worked example | |
|---|---|
| Quoted premium | ₹200 per unit |
| Lot size | L units (use the current figure) |
| Cost of one lot | ₹200 × L |
| If L = 75 | ₹15,000 |
| If L = 50 | ₹10,000 |
The point of showing it this way is that the arithmetic never changes even when L does. Substitute whatever the current lot size is and the calculation holds — which is more useful than a number that was correct the month this page was written.
For an option you are selling
Premium received is calculated the same way, but the cash that matters is the margin blocked, not the premium collected. Option selling margin is set by the exchange's risk model and moves with volatility, so it is substantially larger than the premium and it is not a fixed multiple of it.
Contract value, which is different again
Contract value is the index level multiplied by the lot size. It is what the contract notionally controls, not what it costs you. An option on a contract worth several lakh rupees might cost a few thousand in premium — that gap is leverage, and it is the reason options can be both efficient and dangerous.
Why the exchange changes it
Lot sizes are revised so that the contract value stays within a target range as the index level moves. If the index doubles over a few years and the lot size stays fixed, the contract becomes twice as expensive to trade and effectively excludes smaller participants. Reducing the lot size restores accessibility; raising it does the opposite.
Regulation also plays a part. Where the intent is to keep very small accounts out of leveraged index derivatives, raising the minimum contract value is the lever that does it — and it is a more precise instrument than most alternatives.
How a revision rolls out
Revisions are announced in advance and usually take effect from a specified contract series rather than overnight. Existing contracts generally run to expiry on the old size while new series are issued on the new one, which means both can briefly coexist.
What a change does to your trading
Three things move when a lot size is revised, and the third catches people out:
- Capital per trade changes. A larger lot means more premium outlay and more margin for the same one-lot position.
- Your P&L per point changes. Profit and loss scale directly with lot size, so the same index move is worth more or less in rupees than it was.
- Your historical backtests describe a different instrument. A backtest run under the old lot size shows rupee figures that no longer correspond to a one-lot position today.
That third point matters more than it sounds. If you are reading a backtest report and converting max drawdown into rupees to decide whether you can stomach it, a lot-size change between the test period and today makes that conversion wrong. Re-run the backtest rather than mentally adjusting it.
Freeze quantity, the limit people meet by surprise
Separately from lot size, exchanges cap how many lots can go in a single order. Above that cap the order is rejected rather than queued.
For a retail-sized position this never comes up. For a larger account it does, and the symptom is a rejection that mentions quantity or freeze limits. The fix is splitting the order into several smaller ones, which changes your fill profile and is worth testing before you need it.
How a lot-size change can trigger this
A lot-size revision can push you over a freeze limit you previously sat under, without you changing anything about the strategy. It is worth re-checking after any revision if you trade size.
Position sizing with a fixed step
Because you cannot trade a fraction of a lot, the usual risk-management advice — risk a fixed small percentage of capital per trade — collides with reality on a small account. One lot may already represent more risk than your rule allows.
Three honest responses, in order of preference:
- Trade a strategy whose per-lot risk fits your capital. A defined-risk spread caps the loss in a way a naked short does not.
- Wait until your capital supports one lot at your risk limit. Unexciting, and correct.
- Paper trade until then. Paper trading costs nothing and the strategy learning transfers completely.
A worked sizing example
Numbers make this concrete. Take a trader with Rs.3,00,000 who wants to risk no more than 2% — Rs.6,000 — on any single trade, and who is considering a short straddle.
- Find the lot size. Call it L. The broker order window shows it.
- Find the margin. For a two-leg short straddle the broker shows a combined requirement. Suppose it is Rs.1,40,000.
- Check affordability. Rs.1,40,000 against Rs.3,00,000 is affordable, with headroom for the requirement to rise if volatility does.
- Now check risk, which is the step people skip. If the per-leg stop loss is 30 points and L is 75, each leg risks 30 x 75 = Rs.2,250. Two legs is Rs.4,500.
- Compare against the limit. Rs.4,500 is inside the Rs.6,000 rule, so one lot fits.
What a lot size change does to that
Now change L to 120 and re-run step four: 30 x 120 x 2 = Rs.7,200, which breaches the Rs.6,000 rule. The strategy has not changed and the account has not changed — only the lot size — and the position is now too large.
Where lot size shows up on Arthalab
When you build a strategy you set lots per leg, not units. The engine converts that to the exchange quantity at execution using the lot size in force at the time, so a revision does not silently change what your strategy intends.
Backtests use the lot size applicable to each historical date, which is why a report spanning a revision is still internally consistent. It also means the rupee figures reflect a real historical position rather than today's size applied retroactively. Backtests differ from live results for several reasons — lot size handling is deliberately not one of them.
1
Check the current lot size before sizing
Your broker's order window shows the quantity for one lot. That is the number to plan against.
2
Compute the real cash requirement
Premium times lot size for buying; the broker's margin figure for selling.
3
Confirm it fits your risk rule
If one lot breaches the rule, the strategy is too large for the account, not the other way round.
4
Re-run backtests after a revision
Rupee figures from before a change describe a differently sized position.
5
Re-check freeze limits if you trade size
A revision can move you across a cap you previously sat under.
The short version
Lot size looks like an accounting detail and quietly governs your position sizing.
- Your minimum position is one lot, and sizing moves in whole-lot steps
- Premium times lot size for buying; the broker's margin figure for selling
- The exchange revises lot sizes, so check the current number rather than an article
- A revision changes your rupee risk per trade without the strategy changing
- Re-run backtests after a revision rather than adjusting the figures mentally
If one lot already breaches your risk rule, the strategy is too large for the account. That is a sizing problem, not a reason to relax the rule.
Frequently asked questions
Your broker's order window shows the quantity for one lot, and the NSE or BSE contract specification is the formal source. Both are current in a way an article cannot be.
Yes, directly. A larger lot means more units in the position, so both premium outlay and margin scale with it.
Because risk is stop loss in points multiplied by units. The same 30-point stop risks more rupees on a larger lot, which can push a position outside a risk rule it previously satisfied.
It is revised periodically by the exchange, so the reliable sources are the NSE contract specifications or the quantity shown in your broker's order window. Any fixed number in an article is only correct until the next revision.
Exchange-traded derivatives are standardised into fixed contract sizes. One lot is the minimum tradeable unit, and all orders must be whole multiples of it.
For buying, multiply the quoted premium by the lot size. For selling, the relevant figure is the margin your broker blocks, which is set by the exchange risk model and is much larger than the premium.
Lot size is the number of units in one contract. Contract value is the index level multiplied by the lot size — what the contract notionally controls, not what it costs you.
Yes, in the sense that rupee figures from a period under a different lot size describe a differently sized position. Re-run the backtest rather than adjusting the numbers mentally.
An exchange cap on how many lots can go in one order. Above it the order is rejected, and the usual answer is splitting into smaller orders.
No. They are separate contracts on separate exchanges with independently set lot sizes. Check each one's current specification.
Option selling margin moves with volatility, independently of lot size. A rise in volatility raises the requirement on the same position.
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