Short answer: find the margin rate (percentage) your broker requires, multiply it by the notional value of the position to get the required initial margin, and compare that to your available buying power. Also check maintenance margin and monitor margin level to avoid margin calls.

Detailed step-by-step guide (with formulas and examples)

1. Identify the instrument and broker margin rules

• Different asset types use different rules: cash equities (margin lending/pledge), intraday leverage, futures, options, forex/CFDs.
• Get the broker’s required initial margin (or margin percentage / leverage) and maintenance margin. Example values you might see: initial 50% for some stocks, 3.33% (30x) for forex, or an exchange-specified percentage for futures.

2. Calculate the notional value of the trade

• Notional = Price per unit × Quantity (or contract size × number of contracts). Example (stocks): buy 1,000 shares at ₹200 → Notional = 1,000 × 200 = ₹200,000.

3. Calculate initial margin required

• Formula: Initial margin = Notional × Initial margin rate. Example: broker initial margin = 25% → Margin required = 200,000 × 0.25 = ₹50,000.

4. Check maintenance margin (ongoing requirement)

• Maintenance margin = Notional × Maintenance margin rate.
• If account equity falls below maintenance margin, you may get a margin call. Example: maintenance rate = 20% → Maintenance = 200,000 × 0.20 = ₹40,000.

5. Compute free margin / buying power

• Equity = Cash + Unrealized P/L + other assets.
• Free margin = Equity − Used (initial) margin.
• Buying power depends on leverage: Buying power = Equity × Leverage (or 1 / initial margin rate × Equity). Example: if you have ₹60,000 cash and no other positions:
  • Equity = ₹60,000.
  • Required initial margin = ₹50,000 → Free margin = ₹10,000.
  • You cannot open a second identical position because you lack a further ₹50k.

6. For leveraged instruments (forex/CFDs) — use leverage or margin ratio

• If leverage = 50:1, initial margin rate = 1/50 = 2%.
• Example: buy USD/INR notional ₹500,000 at 50:1 → Margin required = 500,000 × 0.02 = ₹10,000.

7. Futures / options specifics

• Futures: margin is usually set by the exchange + broker add-ons. Use contract size × futures price × margin rate. Example: contract size 100, futures price ₹1,000, margin rate 10% → Notional = 100×1,000 = ₹100,000 → Margin = ₹10,000.
• Options: buying options requires full premium (no margin for buyers). Writing (selling) options requires margin calculated by exchange/broker (often complex: max of certain formulas). Always check the broker/exchange calculator.

8. Margin level and margin call calculations

• Margin level (%) = (Equity / Used margin) × 100.
• If margin level falls below the broker’s maintenance threshold (or stop-out level), positions are liquidated. Example: Used margin = ₹50,000, Equity drops to ₹35,000 → Margin level = (35,000 / 50,000)×100 = 70%. If maintenance threshold is 75%, you’ll get a margin call.

9. Position-size method using risk per trade

• Decide risk% of equity you will lose if stop is hit (e.g., 1% of account).
• Calculate position size so that (entry − stop) × position size = risk amount. This ensures you don’t over-leverage even if margin allows a larger position.

10. Practical checklist before placing a trade

• Confirm initial and maintenance margin rates with your broker for that instrument.
• Compute notional value and margin required.
• Ensure Equity ≥ initial margin + buffer (recommend 10–30% extra to avoid margin calls).
• Calculate worst-case P/L and effect on margin level.
• If writing options, double-check complex margin formulas and required collateral.
• Consider intraday vs overnight margin differences (many brokers allow higher intraday leverage).

Quick example summary

• Buy 500 shares @ ₹150, broker initial margin 30%:
  • Notional = 500×150 = ₹75,000.
  • Initial margin = 75,000×0.30 = ₹22,500.
  • Maintenance (20%) = 75,000×0.20 = ₹15,000.
  • If your account cash = ₹25,000, you can place this trade (free margin after = ₹2,500).

Warnings and best practices

• Always use broker-provided margin calculator when available — it reflects fees, exchange margins, and instrument-specific rules.
• Remember margin magnifies gains and losses; use stop-loss and sensible position sizing.
• Be aware of intraday vs overnight margin differences, and extra requirements for short-selling or option writing.
• Keep a buffer above required margin to avoid forced liquidation during volatility.

If you tell me which market/instrument (Indian equities, NSE futures/options, forex/CFD, or a specific broker), I can give a concrete calculation using typical numbers and a ready-to-use formula.