Free Stock Data APIs Compared | AxionQuant

Free Stock Data APIs

Every financial data API advertises a free tier. What almost none of them advertise is what the free tier actually lets you build. A free plan that caps you at 25 requests per day sounds generous until you realize that a single backtest across 30 tickers will take you two days to complete.

This comparison cuts through the marketing language. It examines the free tiers of the leading stock data APIs on the metrics that actually determine whether you can build something real: rate limits, data quality, historical depth, asset coverage, and the hidden costs that show up after you have already written your integration code.

At a Glance

The table below summarizes the free tiers of the six leading developer-focused stock data APIs. The differences are significant, and they compound as your project grows.

Free Tier Feature Checklist

Rate Limits and Quotas

Rate limits are the first constraint you will hit on any free tier. The models vary widely: some providers use per-minute limits, others use per-day caps, and some use both. The distinction matters because a per-minute limit allows bursty workloads, while a per-day cap forces you to spread work across multiple days regardless of how fast your code runs.

The backtest problem: To backtest a strategy across 30 stocks for one year of daily data, you need 30 API calls. On Alpha Vantage free tier, that is more than your entire daily quota of 25. On Polygon.io free tier at 5 calls per minute, you can complete the same task in 6 minutes. The same workload that takes Alpha Vantage users two days takes Polygon users six minutes.

Finnhub and Polygon.io use per-minute limits without a daily cap, which means your total daily capacity depends on how long you are willing to let your code run. Finnhub free tier allows 60 requests per minute, which at full utilization works out to 86,400 requests per day in theory. Polygon.io free tier allows 5 requests per minute, which caps at 7,200 requests per day.

What Each Rate Limit Actually Allows

Rate limits only matter in relation to what you are trying to build. The table below translates each free tier limit into the concrete workloads it can and cannot support.

Data Quality on Free Tiers

Free tiers vary not just in how much data they give you, but in how reliable that data is. Independent testing has surfaced significant quality differences across providers, particularly around corporate action adjustments and intraday data.

Alpha Vantage free tier has been found to match reference sources on approximately 91 to 98 percent of data points in independent testing, with errors concentrated in adjusted close prices for stocks that had complex corporate actions such as spinoffs and special dividends[reference:0]. The free tier also returns a 15-minute delay on stock data, and the compact output mode returns only the last 100 data points by default[reference:1].

Finnhub free tier provides real-time US stock quotes with a 15-minute delay, but historical OHLCV data is not available at all on the free plan. The free tier also has coverage gaps for smaller caps and OTC stocks, and news data on the free tier has limited historical coverage and restricted outlet diversity[reference:2].

Polygon.io free tier delivers end-of-day data only with 2 years of historical depth and a 15-minute delay on quotes[reference:3]. Index tickers such as VIX and TNX are not available on the free tier at all[reference:4].

Tiingo free tier provides adjusted historical equity data with strong adjustment handling, but intraday depth is limited and the free plan does not include access to the fundamental data API or the Tiingo News API[reference:5].

Twelve Data free tier includes real-time data for US markets, forex, and cryptocurrencies, with 800 API credits per day. However, fundamentals sit in higher Pro and Ultra tiers, so fundamental data is not available on the free plan[reference:6].

Why adjustment errors matter: A one-cent difference in an adjusted close price may seem trivial, but it compounds over time. A backtest that relies on adjusted prices for split and dividend calculations will produce different results depending on which provider you use. For research that needs to be reproducible, data quality is not a secondary concern.

Hidden Costs of Free Tiers

Free tiers have costs beyond the obvious ones. The time you spend working around rate limits, the engineering effort required to handle inconsistent data, and the eventual upgrade price all factor into the true cost of a free plan.

The Engineering Cost

Every workaround you write for a free tier limitation is code you have to maintain. Caching layers, retry logic, exponential backoff, and data normalization all add up. A free tier that saves you $50 per month but requires 20 hours of engineering work to use effectively is not actually free.

Upgrade Cost Over Time

The real cost of a free tier is what you pay when you outgrow it. The table below shows cumulative spend at the entry paid tier over common time horizons.

The legal risk: Some free APIs scrape data from public websites or use undocumented sources without permission from the original providers. This can violate terms of service, copyright laws, and exchange licensing agreements. If your application depends on data from an unlicensed source, you may be exposed to legal risk regardless of what the free tier advertises. AxionQuant sources its data through proper licensing channels, which removes this concern.

Historical Depth on Free Tiers

Historical depth determines what you can backtest and how far back your research can go. Free tiers vary dramatically here, and the differences are not always obvious from the marketing pages.

Who Should Choose Which

No single free tier is best for every use case. The table below maps common scenarios to the provider that fits best.

Verdict

Free tiers are not created equal. Some are genuinely useful for real development work. Others are demo-tier plans that break the moment you try to build anything substantial.

Alpha Vantage remains the most recognizable name in free financial data, but its 25-call daily limit makes it unsuitable for any workload that involves more than a handful of tickers. It works for learning the basics, not for building.

Finnhub offers the most generous per-minute limits and the best real-time access on a free tier, but the absence of historical OHLCV data is a fundamental limitation for backtesting or any research that needs to look backward.

Polygon.io free tier is a demo tier in practice. Two years of EOD data and 5 requests per minute is enough to evaluate the API, not to build on it.

Tiingo and Twelve Data occupy a middle ground. Tiingo offers excellent EOD history and generous daily limits, but limited intraday and no alternative data. Twelve Data provides real-time data and broad asset coverage, but fundamentals are gated behind paid tiers.

AxionQuant is the only free tier in this comparison that provides real-time data, deep historical coverage, alternative data, and commercial licensing without requiring an upgrade. It offers:

If you are evaluating free stock data APIs for a project that needs to go beyond the prototype stage, AxionQuant is the free tier that does not force you to choose between cost and capability.

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