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Forward Testing and Paper Trading

Paper Trading: Realistic Slippage and Execution Costs

Pomegra Learn

Why Does Slippage Matter in Paper Trading?

Slippage is the difference between the price you expected to receive and the price you actually received when you executed an order. It is the cost of buying and selling in an imperfect market. Many trading strategies look brilliant in backtests because they assume perfect fills—you enter exactly at the quote price with zero friction. Paper trading is where you discover whether your strategy survives when slippage and real execution costs are added.

This is not academic. A strategy that shows 12% annual return in backtest with assumed 0.5% slippage might show 3-4% annual return in live trading if actual slippage is 1.5%. A strategy that breaks even after costs is worthless. The difference between a great strategy and a mediocre one is often just execution costs. Paper trading with realistic slippage assumptions is how you discover whether your edge is real or an illusion created by bad assumptions.

Quick definition: Slippage in paper trading is the realistic cost of executing orders—the gap between quoted prices and actual fill prices, including market spread, market impact, commission, and execution delays.

Key takeaways​

  • Slippage includes the bid-ask spread, market impact (size effect), broker commission, and execution timing costs
  • Realistic slippage ranges from 0.1% on liquid large-cap assets to 2-5% on illiquid or small-cap stocks
  • Most traders underestimate slippage; they assume 0.5% but face 1.5-2% in practice
  • Paper trading with unrealistic fill assumptions (too-tight slippage) teaches you false discipline and broken strategies
  • Different order types slip differently: market orders slip most, limit orders slip least (or don't fill)
  • Time of day, asset liquidity, and position size all affect slippage—configure paper trading to reflect this
  • The only way to know your actual slippage is to forward-test, measure it, and compare to published broker statistics

The components of slippage​

Slippage is not a single thing—it is several costs that combine:

1. Bid-ask spread​

The bid-ask spread is the cost of immediate execution. If Apple is quoted at bid $190.50 and ask $190.52, the spread is $0.02 per share. When you place a market buy order, you get filled at the ask ($190.52). When you place a market sell order, you get filled at the bid ($190.50). The round-trip cost (buy and sell) is $0.04 per share, or about 0.02% on a $190 stock.

On large-cap, liquid stocks and major futures contracts, spreads are tight: 1-5 cents on stocks, a few ticks on futures. On illiquid or small-cap stocks, spreads can be 5-20 cents or wider. On thinly-traded options or forex pairs, spreads can be 0.5-2% of the price.

2. Market impact​

Market impact is the price movement caused by your order. When you place a large order, you are demanding liquidity from the market makers. They widen the spread or push the price against you to compensate. A 1,000-share order on a liquid stock like SPY has minimal impact. A 50,000-share order on a less-liquid small-cap stock can move the price 1-2% against you.

The impact is stronger on:

  • Smaller stocks (lower total float, fewer market makers)
  • Illiquid hours (pre-market, after-hours, late-day)
  • Volatile markets (market makers are more defensive)
  • Directional imbalances (everyone is buying, no one is selling)

3. Commission​

Commission is the fee your broker charges per trade. Many brokers now charge zero commission on stock trades, but some still charge $5-10 per trade, or a percentage of the trade value. Options traders often pay $0.65 per contract. Futures traders pay $3-10 per round-trip contract.

On a $10,000 trade, a $10 commission is 0.1%. On a $1,000 trade, it's 1%. Commissions matter more on small accounts and short-term trading.

4. Timing slippage​

Timing slippage is the price movement between when you decided to enter and when your order actually filled. You identify a signal at 10:05:00 AM and place an order. Processing delay, broker routing, and market matching take 0.5 seconds. By the time your order reaches the exchange at 10:05:00.5, the price has ticked against you. On volatile stocks or during news events, a 0.5-second delay can cost 0.5-1%.

High-frequency traders obsess over this; retail traders experience it less but it still adds up.

Realistic slippage by asset class and liquidity​

Here are typical slippage ranges for common assets during normal trading hours:

Large-cap stocks (Apple, Microsoft, Tesla, etc., on NYSE/NASDAQ):

  • Market order slippage: 0.05-0.15% (tight spreads, huge liquidity, but market impact on large orders)
  • Limit order fills: Good, usually fill near the quote
  • Best broker execution: Interactive Brokers, Tastytrade (good routing, low impact)

Mid-cap and small-cap stocks (<$5B market cap, lower volume):

  • Market order slippage: 0.3-1.0% (wider spreads, fewer market makers, more impact)
  • Limit order fills: Less reliable, may not fill during the day
  • Harder to estimate; depends heavily on the specific stock

Major index futures (ES, NQ, GC, crude oil):

  • Market order slippage: 0.05-0.2% (extremely tight spreads, massive liquidity, but overnight and news gaps)
  • Limit orders: Very reliable
  • Best execution: Interactive Brokers, TD Ameritrade (low routing costs, fast connections)

Stock options:

  • Market order slippage: 0.5-2.0% on tight spreads; 2-5% on wider spreads (spreads are much wider than stocks, heavily dependent on volume and Greeks)
  • Limit orders: Critical; always use limit orders on options to avoid terrible fills
  • Bid-ask spread dominates the cost here

Forex (major pairs like EUR/USD):

  • Market order slippage: 0.01-0.05% (tiny spreads, enormous liquidity, but latency-dependent for retail)
  • Limit orders: Fast fills on major pairs
  • Highly time-of-day dependent; Asian and NY session hours are tightest

Crypto (Bitcoin, Ethereum, major exchange):

  • Market order slippage: 0.1-0.5% on major exchanges (tight spreads, variable liquidity)
  • Slippage: Highly exchange-dependent; some exchanges are far worse
  • Highly volatile; slippage increases during news and volatility events

Lower-liquidity assets (penny stocks, illiquid options, exotic forex pairs):

  • Market order slippage: 2-5% or worse (massive spreads, sparse market makers, high impact from any order)
  • Avoid if possible; if required, use limit orders and accept that fills may not arrive

How to configure realistic slippage in paper trading​

Your paper trading platform should have slippage settings or assumptions. Here's how to configure them:

Step 1: Determine your actual broker's typical slippage​

Contact your broker or check their published execution statistics. Most major brokers publish these:

Step 2: Match your paper platform's assumptions​

Most paper trading platforms have a setting for slippage. This is often called "assumed slippage," "execution slippage," or "fill cost." Set it to match your broker's typical rate. If your broker shows average slippage of 1.2% on your asset class, set paper trading to 1.2%.

If your paper platform does not have a slippage setting, or if it uses a fixed rate that doesn't match reality, you must manually adjust your results. For example:

  • Your paper trading backtest shows 8% annual return
  • Assumed slippage in the paper platform: 0.3%
  • Actual broker slippage: 1.0%
  • Difference: 0.7% per trade
  • Rough estimate of impact: Every few trades, you lose an additional 0.7% per trade in execution. Over 100 trades per year, that's 70-100 pips of additional loss.
  • Revised expected return: 8% - 1% = 7% (rough estimate; actual impact depends on your strategy's trade frequency and position holding time)

Step 3: Test with real broker fills during early trades​

During your first 5-10 paper trades, manually compare your paper fills to what a live broker would have charged. Place a paper order at the same time you would in live trading and note the fill. Then, check what the actual spread was on the exchange at that moment. Is your paper simulator filling you at the mid-quote (unrealistically good) or at the ask/bid (realistic)? Adjust assumptions if needed.

Step 4: Vary slippage by order type and time​

If your platform allows:

  • Market orders: Use the higher slippage estimate (0.5-1.5% depending on asset)
  • Limit orders: Use lower slippage (0.1-0.3%), but account for the fact that limit orders don't always fill
  • Pre-market and after-hours trading: Increase slippage by 50-100% (liquidity is much lower)
  • News events or high-volatility periods: Increase slippage by 50-100%

Decision tree​

Real-world slippage examples and impact​

Example 1: The large-cap equity swing trader​

Strategy: Buy 1,000 shares of SPY when it breaks above a 20-day high, hold 2-5 days, exit when a 5-day moving average crosses below price.

Backtest assumptions:

  • Entry slippage: 0.2%
  • Exit slippage: 0.2%
  • Commission: $0 (interactive brokers, commission-free)
  • Annual trades: 30 trades (one every 12 days)

Backtest result: 9.5% annual return

Paper trading with realistic Interactive Brokers fills:

  • Entry slippage (market orders on SPY at 10 AM): 0.05% (very liquid)
  • Exit slippage (market orders on SPY, end-of-day): 0.08%
  • Commission: $0
  • Actual result: 9.2% annual return (minimal impact because SPY is very liquid)

Lesson: Large-cap liquid stocks are forgiving. Slippage assumptions matter less. The strategy is robust.

Example 2: The small-cap reversal trader​

Strategy: Buy stocks that gap down >3% intraday, expecting reversal, exit at end-of-day.

Backtest assumptions:

  • Entry slippage: 1.0% (attempting to catch the dip quickly)
  • Exit slippage: 1.0%
  • Commission: $5 per trade (older broker)
  • Average trade size: $5,000 (100 shares at $50)
  • Annual trades: 60 trades

Backtest result: 6.5% annual return

Paper trading with realistic Interactive Brokers fills on small-cap stocks:

  • Entry slippage (market orders during volatile intraday): 2.0-3.0% (wide spreads, low volume, market impact)
  • Exit slippage (end-of-day market order): 1.5%
  • Commission: $0 (commission-free)
  • Total slippage per trade: 3.5-4.5%
  • Round-trip impact: ~4.5% per trade on a $5,000 trade
  • After 60 trades: $300-450 in total slippage (6-9% of starting capital)

Actual result: 1-2% annual return (strategy edge is almost entirely consumed by execution costs)

Lesson: Small-cap, illiquid stocks have massive slippage. A strategy that looks good in backtest with 1% slippage assumption is worthless when realistic slippage is 3-4%. The trader must either find a better asset class or use limit orders (and accept fewer fills).

Example 3: The options credit-spread seller​

Strategy: Sell 1-standard-deviation out-of-the-money call spreads on SPY weekly, collect premium, exit before expiration.

Backtest assumptions:

  • Entry slippage: 0.5% of premium collected (bid-ask spread on options)
  • Exit slippage: 0.5%
  • Commission: $0.65 per contract per side ($1.30 round-trip per spread)
  • Average premium collected per trade: $100-150
  • Annual trades: 50 trades (weekly)

Backtest result: 11% annual return

Paper trading with realistic Tastytrade fills (options broker):

  • Entry slippage on 1-standard-deviation SPY spreads: 1.5-2.5% of premium (wider spreads on options than stock, bid-ask is larger)
  • Exit slippage: 2-3%
  • Commission: $0.65 per contract per side = $1.30 per spread (50 trades/year = $65 annual commission)
  • On $100-150 per trade, slippage is $2-4 per trade, commission is $1.30
  • Total cost per trade: $3-5 (2-5% of premium collected)
  • Actual result: 7-9% annual return (realistic)

Lesson: Options slippage is substantial. The bid-ask spread is the largest component. The strategy still works, but with more realistic return expectations.

Common slippage mistakes and how to avoid them​

Assuming zero commission in backtests when your broker charges. You do a backtest with commission set to $0 because your broker is commission-free on stocks. You then decide to trade options with the same broker (which charges $0.65/contract) or you switch brokers and the new broker charges. Your live results are immediately worse than backtest. Solution: research your actual broker's commission schedule and always include it in backtests and paper trading.

Using a paper trading simulator with unrealistically tight fills. Some simulators fill you at the midpoint with zero slippage. You forward-test, think your strategy is amazing, then go live and discover your edge evaporates. Solution: verify your paper platform's fill assumptions before you begin. Compare fills to published broker statistics. If the platform is too optimistic, add manual slippage adjustment (multiply P&L by 0.7 or 0.8 to estimate the impact).

Ignoring time-of-day slippage variation. You paper-trade during liquid market hours (9:30-11:30 AM ET) and see tight slippage. You then take trades during low-liquidity periods (pre-market, late afternoon, after-hours) and experience 2-3x worse slippage. Solution: track your slippage separately by time-of-day during paper trading. If your strategy trades at specific times, use those times in paper trading to get realistic fills.

Underestimating slippage on large orders. Your paper platform assumes you can buy 10,000 shares of a small-cap stock at the quote price. Reality: that order moves the market 1-2% against you because the stock only trades 5,000 shares per hour. Solution: size your paper trades to match your intended live position size. If you'll trade 1,000-share orders live, paper trade 1,000-share orders. Do not scale up or down just because it's paper.

Changing slippage assumptions mid-strategy. You start paper trading with 1% slippage assumption, but your first five trades are tight fills at 0.3% slippage. You get optimistic, reduce your slippage assumption to 0.5%, and finish forward testing with better results. Then live trading shows the edge is gone. Solution: lock in slippage assumptions before you start forward testing. Do not adjust mid-stream based on a few lucky fills.

Ignoring volatility-dependent slippage. During normal market conditions, slippage on your asset is 0.5%. During the Fed announcement or earnings, slippage spikes to 1-2% as spreads widen. Your backtest and early paper trading happen during calm markets, so you assume 0.5% for all conditions. Then you trade during volatile periods and get destroyed. Solution: review your strategy's exposure to high-volatility periods. Adjust slippage assumptions higher if you trade during news events or volatile hours.

FAQ​

How do I know if my paper trading slippage is realistic?​

Compare your paper fills to your broker's published execution statistics. If your paper platform fills you consistently at the mid-quote with zero spread, the slippage is too optimistic. Most realistic slippage includes at least half the bid-ask spread plus market impact. Check https://www.finra.org/rules-guidance-oversight-enforcement for market-wide execution quality or your specific broker's published stats.

Should I set slippage differently for different assets in my strategy?​

Yes, if possible. Large-cap stocks might use 0.15% slippage; small-cap stocks might use 1.0%. Options might use 2.0%. If your paper platform doesn't support variable slippage by asset, use a conservative (high) estimate that applies to all trades.

What if I switch brokers?​

Verify your new broker's slippage statistics. Different brokers have different routing quality, which affects fills. Interactive Brokers is generally tight; smaller brokers sometimes have worse execution. Update your paper trading slippage assumptions if your broker's typical execution differs significantly.

Is slippage the same on limit orders as market orders?​

No. Limit orders don't have immediate slippage—they either fill at your limit price or don't fill. However, they have a different cost: you miss the trade if the price never reaches your limit, or you have a much-delayed fill if the market moves slowly to your target. In paper trading, use market order slippage assumptions for market orders and zero slippage (but with fill-rate risk) for limit orders.

How much should I increase slippage during volatile or news-driven markets?​

Increase by 50-100%. If your typical slippage is 0.5%, assume 0.75-1.0% during Fed announcements, earnings seasons, or geopolitical events. If your typical slippage is 1.5%, assume 2.5-3.0% during high-volatility periods.

Can I ignore slippage if I'm trading very large accounts?​

No. Slippage becomes more important on larger accounts, not less. A 1% slippage cost on a $1 million account is $10,000 per trade. Scale matters; larger traders often have slightly better execution (prime broker pricing), but they also face larger market impact. Assume slippage is approximately scale-independent in percentage terms.

What if my strategy mostly uses limit orders?​

Limit orders have different costs: tight fills when they execute, but some orders don't fill at all. In paper trading, estimate a fill rate (say, 70% of limit orders fill), apply zero slippage to filled orders, and assume unfilled limit orders miss the trade entirely (creating opportunity cost). This is more complex; many traders simplify by using market order slippage as a conservative estimate even for limit orders.

Summary​

Realistic slippage in paper trading accounts for bid-ask spreads, market impact, commission, and timing costs. Most traders underestimate slippage, assuming 0.5% when actual execution costs are 1.5-2.0%. By configuring paper trading with broker-specific fill assumptions, testing against published execution statistics, and adjusting for asset class and time-of-day variation, you ensure that paper-trading results predict live trading results. A strategy that does not survive realistic slippage assumptions is not a strategy—it is a backtest artifact. Paper trading with honest slippage is the final test of whether your edge is real.

Next​

Paper Trading: Using Real Rules