Bitcoin closed August with a red monthly candle, the first after four consecutive green months. September holds critical importance as it marks the end of Q3 and provides key price signals for analysts to forecast Bitcoin’s outlook toward year-end.
Which factors could shape Bitcoin’s price action this month? The following outlines four main drivers to watch.
1. ETF inflows may repeat early-year pPatterns
Bitcoin ETF flows in the US strongly correlate with Bitcoin’s price movements. Analyst Yonsei_dent recently observed that ETF inflows over the past two months resemble the dynamics seen at the start of 2025.
Bitcoin Price vs. Bitcoin ETF Fund Holdings. Source: CryptoQuant
Despite a price decline in January–February 2025, ETF holdings stayed relatively stable. Once ETF balances began to drop sharply, Bitcoin’s price corrected significantly in tandem.
Yonsei_dent compared this to the last two months and warned that September may be difficult for Bitcoin:
More recently, we’re seeing a similar structure playing out in July–August. Bitcoin has pulled back after setting a new ATH, but ETF holdings are still holding steady — for now. If we begin to see a deeper outflow of capital from ETFs, Bitcoin could face further downside pressure.” Yonsei_dent predicted.
Additionally, SoSoValue data shows that on August 29, US Bitcoin ETFs recorded a net outflow of more than $126 million. This ended a four-day positive streak and signaled weakness in ETF capital flows.
2. Whale-driven BTC selling and ETH accumulation
A notable shift in whale behavior is a second factor shaping Bitcoin’s September trend. As revealed by on-chain transactions, large holders have started selling Bitcoin and accumulating Ethereum (ETH).
For example, Lookonchain reported that a Bitcoin OG sold 4,000 BTC ($435 million) and bought 96,859 ETH ($433 million) on the last day of August. On September 1, the same whale sold another 2,000 BTC ($215 million) and purchased 48,942 ETH ($215 million). In total, this address has accumulated 886,371 ETH ($4.07 billion).
This wave of BTC selling and ETH buying reflects a change in investor expectations between the two assets. The transparency of these on-chain moves can influence market sentiment and trigger similar behavior among retail investors. As a result, the trend could shape both BTC and ETH price action in September.
3. US demand measured by Coinbase Premium Index
The Bitcoin Coinbase Premium Index tracks the price gap between the US market (Coinbase) and the global market (Binance).
When the index is positive, it signals stronger buying demand from US investors, often driving Bitcoin prices higher.
The index fell from 100 to 11.6 at the start of September. Although still positive, the decline suggests weakening US buying momentum.
Bitcoin Coinbase Premium Index. Source: CryptoQuant
Monitoring this indicator helps identify early support signals. If the index turns negative, caution is warranted, especially when combined with weakening ETF inflows and whale-led BTC selling.
However, XWIN Research Japan maintains optimism when pairing this metric with Delta Cap:
Bitcoin consolidating above $100,000 with strong institutional support and a long-term valuation floor steadily rising. Corrections, rather than being a sign of weakness, appear to be opportunities for accumulation within a robust structural uptrend.” XWIN Research Japan said.
4. Record US M2 money supply and Fed rate-cut expectations
The final, and potentially most powerful, factor for Bitcoin in September is the Federal Reserve’s interest rate decision. Markets anticipate a rate cut in September, which could channel capital into risk assets such as BTC.
“Based on what I know today, I would support a 25 basis point cut,” Federal Reserve Governor Christopher Waller said.
Meanwhile, according to Fred data, the US M2 money supply has reached a record $22.1 trillion. Historically, periods of rising M2 combined with falling interest rates have fueled Bitcoin rallies.
US M2 Supply. Source: Fred
22.1 TRILLION. Fresh ATH for U.S. M2 Money Supply. The Fed may talk ‘tightening,’ but the printer clearly hasn’t retired…BTC is the hedge you front-run, not chase,” investor Kyledoops stated.
September could be a decisive month for Bitcoin, shaped by ETF capital flows, whale transactions, US investor demand, and Federal Reserve policy.
If positive factors dominate, Bitcoin may stage a strong recovery. Conversely, sustained ETF outflows and whale-led selling could extend downward pressure.
Tracking these drivers closely will help traders detect early signals and avoid heavy losses from sudden volatility.