The global ETF industry has been growing by leaps and bounds and has already accumulated almost $4.5 trillion in assets, as per ETFGI data. ETFs gained popularity over mutual funds because of their flexibility, liquidity and low cost among other factors.

In fact, low cost has been one of the biggest drivers for the ETFs, enhancing their total returns. This is primarily because fund managers generally don’t actively manage an ETF. As these products often engage in passive index-based investing, they charge a much smaller fee.

Several research reports have shown that only a handful of fund managers outperform the market over the long term. This gives a major boost to passive investing strategies.
 
As per the 2015 SPIVA U.S. Scorecard, over the last five years, 84.2% of large-cap managers, 76.7% of mid-cap managers, and 90.1% of small-cap managers underperformed the S&P 500, the S&P MidCap 400 and the S&P SmallCap 600, respectively. The number of managers outperforming the benchmark index is equally bleak over the 10-year investment horizon. Roughly 82.14% of large-cap managers, 87.61% of mid-cap managers, and 88.42% of small-cap managers lagged their respective benchmarks. Additionally, managers across all international equity categories failed to outperform their benchmarks in the above mentioned time frame.
 
Although there are several cost components to an ETF like trading commissions and bid/ask spreads, expense ratios are paid the foremost attention by investors. With several ETF providers including iShares, Vanguard and Charles Schwab vying with each other, ETFs have gotten cheaper every year (read: 5 Costly ETF Mistakes You Can Easily Avoid).
 
Knowing how important the expense ratio is, we have highlighted five of the cheapest ETFs for long-term investors (see: all the ETFs with Low Expense Ratios here):
 
iShares Core S&P Total U.S. Stock Market ETF (ITOT)– Expense ratio: 0.03%
 
This fund provides a broad exposure to the U.S. equity market by tracking the S&P Total Market Index and is one of the low-cost choices in the equity ETF world, charging just 3 bps in annual fees. Holding 3,819 securities, the fund is widely diversified across sectors and securities. Information technology is the top sector accounting for less than 20% while Apple (AAPL - Analyst Report) is the top firm taking 2.7% share of the basket. Large caps account for 74% of the assets while mid and small caps take the remainder. ITOT is a popular and liquid ETF with AUM of $3.6 billion and average daily volume of 286,000 shares. The product has delivered 70.9% returns over the last five-year period.
 
Schwab U.S. Broad Market ETF (SCHB​) – Expense ratio: 0.03%
 
This fund also provides a broad exposure to the U.S. equity market. The fund tracks the Dow Jones U.S. Broad Stock Market Index and charges just 3 bps in annual fees. Holding 2,077 securities, the fund is widely diversified across sectors and securities. Like ITOT, information technology is the top sector accounting for less than 20% while Apple is the top firm taking 3.3% share of the basket. Large caps account for 73% of the assets while mid and small caps take the remainder. SCHB is one of the popular and liquid ETFs with AUM of $5.8 billion and average daily volume of 927,000 shares. The product has delivered 70.7% returns over the last five-year period (read: ETF Tactics for a Rate-Proof Portfolio).
 

Schwab U.S. Large-Cap ETF(SCHX) – Expense ratio: 0.03% 

This fund targets the large-cap segment of the U.S. equity market by tracking the Dow Jones U.S. Large-Cap Total Stock Market Index and holds 777 securities in its basket. Here again, information technology is the top sector with just over 20% share while Apple is the top firm at 3%. With an expense ratio of 0.03%, the fund has amassed $5.3 billion in its asset base and volume is solid at over 783,000 shares. While this is a large-cap fund, mid and small caps take minor portions each in the basket. The fund has gained about 72.4% over the past five-year period.
 
Vanguard Total Stock Market ETF (VTI) – Expense ratio: 0.05%
 
This ETF follows the CRSP US Total Market Index, holding a large basket of 3,712 securities. Each security holds no more than 2.5% of total assets while financials, technology, consumer services and health care make up for a nice sector mix in the portfolio. It is one of the largest and a popular fund with AUM of nearly $57.9 billion and average daily volume of nearly 3.5 million shares. It charges 5 bps in fees and expenses and has gained 70.3% over the past five years (read: Top-Ranked Value ETFs & Stocks to Beat a Choppy Market).
 
Vanguard S&P 500 (VOO) – Expense ratio: 0.05%
 
This is another low-cost, well-diversified large-cap fund tracking the S&P 500 index. It holds 505 securities in its basket with each taking less than 3.2% share while sector-wise too, none accounts for more than 21% of assets. The fund has AUM of $43.5 billion and trades in heavy volume of 2.7 million shares per day on average. Expense ratio came in at 0.05%. The ETF returned about 74.5% in the same period.