How a Central Bank Sets the Overnight Interest Rate

A central bank sets the overnight interest rate not by decree but by managing the supply of reserves banks hold and establishing a corridor of permitted rates. By adjusting the total amount of reserves in the banking system, offering lending facilities at published rates, and conducting open market operations, the central bank nudges the overnight interbank lending rate toward its target. The rate is the price at which banks borrow from each other to maintain their required reserve balances, and it ripples through the entire economy—influencing mortgages, business loans, savings rates, and inflation.

The Overnight Rate and Reserve Demand

Banks are required to hold a minimum amount of reserves relative to their deposits—a reserve requirement set by the central bank. At the end of each business day, banks balance their reserve accounts. A bank short of reserves will borrow from a bank with excess cash, and the interest rate they negotiate is the overnight rate.

Individual negotiations would be inefficient, so most overnight lending occurs in the federal funds market (in the US) or similar interbank lending pools. Thousands of transactions happen daily at slightly different rates, so statisticians compute an overnight index—the Fed publishes the effective federal funds rate, a weighted average of all overnight transactions.

The overnight rate is important because banks cannot easily create reserves themselves; they depend on the central bank to supply them. This dependence is the lever.

Reserve Supply: The Fundamental Control

The central bank controls the overnight rate by managing total reserve supply. Think of it this way: if the central bank injects massive amounts of reserves into the system, most banks will have excess cash and be eager to lend overnight; borrowers face a flooded market and rates fall. Conversely, if the central bank drains reserves, scarcity drives rates up as borrowers compete for scarce overnight funds.

The central bank adds reserves through open market operations (OMOs)—typically by purchasing securities (like Treasury bonds) from banks. The bank receives cash in exchange, crediting its reserve account at the central bank. Removing reserves works in reverse: the central bank sells securities or lets maturing bonds roll off its balance sheet.

By carefully calibrating the net supply of reserves, the central bank pushes the overnight rate toward its target without micro-managing individual transactions.

The Corridor and Standing Facilities

To prevent the overnight rate from drifting too far, the central bank establishes a corridor—an upper and lower bound. At the top sits the discount rate, the rate at which the central bank lends reserves directly to banks in need (the Fed’s “discount window”). Banks know they can always borrow at this rate, so very few overnight rates trade above it; the market rate gravitates toward or below it.

At the bottom sits the interest on reserves (IOR) rate, the rate the central bank pays banks on their excess reserves held overnight at the central bank. Banks know they can always deposit excess reserves and earn this rate, so very few overnight rates trade below it; the market gravitates toward or above it.

Between these boundaries, the market-clearing rate reflects actual supply and demand for reserves. The corridor narrows the range of uncertainty, stabilizing overnight rate expectations.

Open Market Operations: The Dial

OMOs are the day-to-day lever. A typical OMO involves the central bank’s trading desk posting a bid or offer for Treasury securities. When the desk buys securities, it injects reserves; when it sells or allows securities to mature without reinvestment, it drains them.

If the overnight rate is running above target, the central bank adds reserves by purchasing securities, flooding the market and pushing rates down. If the rate is below target, the central bank drains reserves by selling or letting bonds mature, creating scarcity and nudging rates up.

The Fed’s desk may also use repurchase agreements (repos): the central bank lends cash to banks in exchange for collateral (usually Treasuries), with the banks agreeing to repurchase later at a slightly higher price. This is a short-term reserve injection. If the Fed wants to drain, it can offer reverse repos (borrowing cash from banks and lending them Treasuries), pulling reserves out temporarily.

When Scarcity Breaks Down: 2019 Repo Crisis

In September 2019, the Fed faced an unusual situation. Despite months of adding reserves, the overnight repo rate spiked to 10% because banks faced temporary funding pressures and hoarded cash. The issue wasn’t total reserves but their uneven distribution; some banks had excess while others faced temporary shortfalls, but the interbank market froze.

The Fed responded by dramatically expanding repo operations, lending directly to the repo market to ensure smooth function. This episode highlighted that reserve supply alone is not enough if the distribution mechanism jams—the Fed also needs to ensure the corridor stays effective and money markets remain liquid.

Transmission to the Wider Economy

The overnight rate itself is a wholesale banking tool, invisible to households. Its power lies in transmission. Banks that borrow overnight at the Fed’s target rate adjust their lending rates to borrowers. A lower overnight target encourages banks to offer cheaper mortgages, auto loans, and business credit lines, spurring spending. A higher overnight target makes borrowing costlier, cooling demand.

Financial markets also react. Bond traders expect overnight rate changes to influence longer-term bond yields over time, so the central bank’s forward guidance—statements about where it plans to take the overnight rate—moves stock and bond prices immediately, even before the Fed acts.

Forward Guidance and Expectations

Modern central banks also steer expectations through forward guidance: public statements about the likely path of future overnight rates. A central bank that commits to keeping rates low “for an extended period” anchors market expectations downward, lowering long-term rates even if the overnight rate itself has not changed.

This is powerful because inflation and spending decisions depend partly on expectations. If households and firms believe rates will stay low, they spend and borrow more aggressively. The Fed and other central banks learned during the 2008 crisis that expectation management is often as important as the current overnight rate itself.

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