Myth: Higher Return Assumptions Are Harmless

⏱ 4 min read · Finance Utils

Why People Use Optimistic Return Assumptions

When running a retirement or long-term goal calculator, higher return assumptions produce more reassuring numbers — larger projected corpuses that seem to confirm you're on track. It feels like being "optimistic." Brokers and fund houses sometimes show historical peak returns (18–20% in strong years) in marketing materials, leading investors to use inflated numbers as baselines. The myth holds that using a higher return assumption is merely being positive, and the worst case is that you end up with a pleasant surprise of more money than projected. This framing is fundamentally wrong.

The Real Harm: Systematic Under-Saving

The actual harm of a high return assumption is not that you end up with less money than projected — it is that you save less than you needed to. If a 10% return assumption shows you reaching ₹1 crore in 15 years with ₹25,000/month SIP, you invest ₹25,000/month. If the actual return is 8%, you reach only ₹86 lakh in 15 years — a ₹14 lakh shortfall. To hit ₹1 crore at 8%, you needed to invest ₹29,000/month from the start. The damage isn't from the return being lower; it's from the lower saving rate that the higher assumption made seem sufficient. Higher return assumptions systematically lead to insufficient savings rates.

A Concrete Numbers Comparison

Goal: ₹1 crore corpus in 15 years. Monthly SIP required at 15% assumed return: ₹14,700. Monthly SIP required at 12% assumed return: ₹19,800. Monthly SIP required at 10% assumed return: ₹25,000. Monthly SIP required at 8% assumed return: ₹29,000. Someone planning at 15% return invests ₹14,700/month, believing it is sufficient. If actual return is 8%, they reach only ₹51 lakh in 15 years — barely half their goal. The 15% assumption didn't just mean falling short by a small margin; it caused a catastrophic planning error by halving the required SIP amount.

What Return Assumptions Are Actually Reasonable

For Indian equity mutual funds over 15–20 year horizons, a conservative planning assumption of 10–11% CAGR is appropriate. This is below the historical CAGR of major Indian indices (Nifty 50 long-term average ≈ 12–13%) but accounts for future uncertainty, sequence-of-returns risk, and the fact that past performance doesn't guarantee future results. For mixed equity-debt portfolios, 8–9% is reasonable. For debt-heavy or conservative portfolios, 6–7%. Using these conservative assumptions means you'll likely end up with more than you planned — which is a pleasant problem to have, unlike the alternative.

How to Build Return Assumptions Into Planning Correctly

Plan at a conservative baseline and stress-test at lower returns. For any goal, calculate the SIP or lumpsum required at your conservative assumption (10% for equity) and treat that as your minimum savings target. Then run the same calculation at a pessimistic assumption (7–8%) and understand the gap — this is the additional buffer you could build. If equity returns come in at 12–13% historically, any excess corpus is a bonus that can be retired earlier or extended to new goals. This conservative-plus-buffer approach produces resilient financial plans that work even in below-average market environments.