Reference

Standard, mini, micro, nano: lot sizes in forex.

A standard lot is 100,000 base-currency units. A mini is 10,000, a micro is 1,000, and a nano (where offered) is 100. The lot size determines pip value linearly — and therefore everything downstream.

The four sizes

Lot size	Units	Pip value (EUR/USD, USD account)	Pip value (USD/JPY, USD account)
Standard (1.0)	100,000	$10.00	~$6.49
Mini (0.1)	10,000	$1.00	~$0.65
Micro (0.01)	1,000	$0.10	~$0.065
Nano (0.001)	100	$0.01	~$0.0065

Pip value scales linearly with lot size: a mini lot has 1/10 the pip value of a standard lot, etc. This is the relationship that lets the position-size calculator scale risk to fit any account size.

Which lot size for which account

Account < $1,000. Micro or nano lots. A 1 % risk per trade on a $500 account is $5 of risk. Standard lots are too large; even mini lots overshoot for typical 30-pip stops ($30 of risk per mini on EUR/USD).
Account $1,000–$10,000. Micro or mini. The increment of risk on micro lots ($0.10 per pip) is small enough to size precisely; mini lots ($1 per pip) work for anything above ~$5,000.
Account $10,000–$100,000. Mini lots primarily. Standard lots become viable above $25,000 with conservative risk per trade.
Account $100,000+. Standard lots. The micro/mini overhead becomes meaningful in execution slippage at this size.

The custom-units case

Some brokers allow position sizing at any unit count (1 unit = 1 base-currency unit). This is the most flexible — you can size to exactly the dollar risk you want without lot-rounding compromises. The trade-off is that order tickets at non-standard unit counts are non-uniform across brokers; switching platforms can require adjusting your tracking spreadsheet.

The calculator's “Custom (units)” lot type accepts any unit count and computes pip value linearly.

Common lot-size errors

Confusing lots with units. “1 lot” on most platforms = 1 standard lot = 100,000 units. On some MT5 configurations, “1 lot” defaults to 1 mini lot. Verify by inspecting the order-ticket position-value field before placing.
Over-sizing on micro accounts. Trading 1.0 lots on a $500 micro account assumes 100,000 units of exposure with $500 backing — effectively 200:1 leverage. A 0.5% adverse move is a margin call.
Under-sizing on standard accounts. The opposite mistake: trading micro lots on a $50,000 account because the platform default is 0.01. Risk per trade ends up tiny; the account never grows materially.

The right discipline: compute the position size from the risk budget (account size × risk percentage) divided by the per-pip dollar value at your chosen lot size, divided by the pip stop. The calculator does this in the “Suggested size for risk budget” output line.

Margin per lot at standard retail leverage

EU retail leverage caps at 1:30 for majors. US retail caps at 1:50 for majors. The required margin per lot at typical EUR/USD pricing:

Lot size	Position value (USD)	Margin at 1:30 (EU)	Margin at 1:50 (US)
Standard (1.0)	$107,759	$3,592	$2,155
Mini (0.1)	$10,776	$359	$216
Micro (0.01)	$1,078	$36	$22
Nano (0.001)	$108	$3.59	$2.16