Lump Sum vs Monthly Investment Calculator

Lumpsum = total × (1 + r)^t. Monthly = future value of the equal monthly instalments. A market dip is modelled as a price path, not a flat rate.

Why the lump sum usually wins

If markets only ever rose at a steady rate, this would not be a contest: investing a lump sum all at once always beats drip-feeding the same money in. The reason is simple — every pound is working from day one, compounding for the full period, while the monthly plan leaves most of the money on the sidelines for years. Historically, investing a lump sum has beaten spreading it out roughly two-thirds of the time.

So with a flat return the calculator will always show the lump sum ahead, and it says so plainly. That is not a quirk of the maths — it is the honest answer. The real argument for investing monthly is not a higher return; it is lower regret if the market falls just after you invest.

Worked example — $120,000 at 10% over 10 years, flat return: Lump sum: 120,000 × 1.10^10 ≈ $311,250. Monthly ($1,000 a month): ≈ $199,900. The lump sum finishes about $111,000 ahead — because it was fully invested the whole time.

When monthly investing wins: the dip scenario

The one situation where monthly investing genuinely comes out ahead is when the market falls soon after you would have invested the lump sum. Switch on the market-dip scenario — an early decline and a recovery period — and the calculator models a price path instead of a flat rate. The lump sum, invested at the top, rides the fall all the way down and back up; the monthly plan keeps buying through the decline at lower prices, accumulating more for the recovery.

How deep the dip has to be may surprise you. Over a long horizon the lump sum usually still wins even after a steep fall, because the crash reverses and its years of full exposure win out — a 40% drop early in a ten-year plan narrows the gap but rarely closes it. Monthly investing tends to win only when the decline is both deep and a large slice of the period, so the cheap purchases dominate. When that happens the two lines on the chart cross, and the calculator reports the moment monthly overtakes.

Worked example — $120,000 over 5 years at 8%, with a 50% crash that takes 4 years to recover: The lump sum is dragged down for years and ends near $176,000. The monthly plan, buying through the lows, ends ahead at about $193,000 — overtaking the lump sum in year 4.

Making it your real comparison

By default both strategies invest the same total, so the contest is purely about timing. Use the monthly override to set your actual monthly amount independently — then it compares a real lump sum against a real monthly SIP that need not add up to the same total. The idle-return option credits the monthly plan’s waiting cash with interest, since money queued for investment usually sits in savings rather than doing nothing.

Inflation and tax can be layered on too; they apply equally to both sides, so they rarely change the winner, but they do show what each strategy is really worth in today’s money and after tax.

The honest takeaway

If you have a lump sum and a long horizon, the evidence and the maths both favour investing it now. If the thought of the market dropping the week after keeps you from investing at all, a monthly plan is a sound behavioural compromise — you give up a little expected return for a lot less regret risk. This tool lets you put numbers to that trade-off rather than guessing.