You can invest a lumpsum amount in a mutual fund. The minimum is just Rs 100 in some liquid mutual funds.
While SIP is a good way to invest with discipline in a scheduled manner, you can also do lumpsum invest as and when money becomes available.
A nice hybrid way is the “Pay as you go” SIP in the Jama Direct Mutual Fund app. This was you will be prompted to invest each month in the SIP; you can pay or ignore based on your fund availability. You can always come back and invest anytime in that mutual fund and add more units to your folio account.
When you invest, ensure you only do in Direct Plan Mutual Funds. You then get upto 40% more in long term portfolio size by avoiding commissions and brokers.
