SIP vs Lumpsum Investing

The mechanics

A lumpsum puts everything in at once. Every rupee compounds for the full duration.

A SIP spreads the same total across many months. Each rupee compounds for a different — and shorter on average — time.

The math

Lumpsum FV = P × (1 + r) ^ n
SIP FV     = M × ((1 + i)^n − 1) / i × (1 + i)

Where M is the monthly amount, i is the monthly rate, and n is the number of months.

When SIP wins

• When you do not have a lumpsum to begin with.
• When markets fall first and recover later (you buy cheaper).
• When you want behavioral discipline more than optimization.

When lumpsum wins

• When markets rise steadily (every later contribution buys at a higher price).
• When you have idle cash that is otherwise earning less.
• When you have already accepted the risk and timing it adds no information.

The real answer

For most people, the decision is set by cash flow, not optimization. If you earn monthly, you invest monthly. If you have a lumpsum, you invest it. The arguments for one or the other usually matter much less than starting and not stopping.