Gemini API Billing Tiers: Prepay vs Postpay and No Credits Fixes

As of October 10, 2026, paid Gemini API usage in Google AI Studio is converging on one billing plan: Prepay. New accounts start on Prepay, and Google is moving existing Postpay developer accounts over. Impress Watch reported the cutover as October 12, 2026; the date that applies to you is the one in your own account notice. Accounts that use only Free Tier features don't need to do anything.

The plan is only half of "billing tier." Every billing account also sits in a usage tier (Free, Tier 1, Tier 2, or Tier 3), and the tier sets a monthly spend cap of $250, $2,000, or $20,000–$100,000+ plus a rolling 10-minute spend limit of $10 on Tier 1 or $200 on Tier 2 and Tier 3. When paid calls stop, one of five controls usually fired: a $0 Prepay balance (HTTP 402), the rolling spend limit or a per-project rate limit (429 RESOURCE_EXHAUSTED), the monthly tier cap (every linked project paused), or a project spend cap.

An API key is only a credential. Before you buy credits or create another key, find which control stopped the request. The table in the diagnosis section below maps each symptom to its fix.

Postpay to Prepay migration: what to do before October 12, 2026

Google's Gemini API billing documentation (last updated September 28, 2026) says AI Studio "is transitioning developer accounts from Postpay to Prepay billing for Gemini API usage" and tells you to "switch to Prepay and add credits before the cutover date in your account notice." The page itself prints no date. Impress Watch's August 13, 2026 report gives October 12 and says service is interrupted for paid accounts that haven't switched and bought credits by then.

If you're on Postpay today, this is the order that avoids a gap:

1. Confirm your plan. In AI Studio, open Projects and read the Billing Tier column, or open the Billing page. If it shows Free, stop here: Free-only accounts need no action.
2. Find your notice. The cutover date in your account notice overrides any date reported in the press, including the October 12 date above.
3. Click "Switch to Prepay" on the AI Studio Billing page. The switch covers Gemini API usage only. Other Google Cloud services on the same billing account stay on Postpay.
4. Buy credits. The minimum purchase is $5 and the maximum balance is $5,000. Credits expire after 12 months and Prepay purchases can't be refunded, so size the first purchase from recent Gemini spend rather than rounding up.
5. Turn on auto-reload. Set a payment method, a reload amount, and the trigger balance, then a monthly auto-charge limit that matches what you're willing to spend unattended. Impress Watch's report also recommends auto-reload.
6. Budget for the January 1, 2027 price change. Gemini 3.8 Flash and 3.6 Flash cost $0.75 input / $3.75 output per 1M tokens through December 31, 2026. From January 1, 2027 the rates double to $1.50 / $7.50. A balance sized for December drains about twice as fast on the same Flash traffic in January.
7. Check invoiced accounts separately. Google's billing page says Prepay "is not available for Invoiced (or Offline) accounts" and doesn't describe their migration path. Ask your Google Cloud billing contact before the cutover date.

Switching is one-way. The current billing FAQ says "switching from a Prepay billing plan to a Postpay billing plan is not supported."

A single report on the Google AI Developers Forum shows what an unfunded switch looks like: a Tier 1 Postpay account said it was moved to Prepay without opting in and began returning 429 errors about depleted prepayment credits. If your account is switched before you fund it, expect that symptom and add credits first.

The 60-second signed-in billing check

Use the same project for every step. Reading the billing screen of one project while debugging a key from another is the fastest way to misdiagnose the problem.

1. Identify the project. In AI Studio's API Keys or Projects page, find the project that owns the key the failing request uses. Don't copy or expose the secret.
2. Read plan and tier. The Projects page's Billing Tier column shows the usage tier and billing plan; the Billing page shows the same plus the Prepay balance.
3. Read the balance. On Prepay, a $0 balance stops every key in every project linked to that billing account.
4. Read the monthly cap and spend. Compare this month's spend with the tier cap: $250 for Tier 1, $2,000 for Tier 2.
5. Read the active rate limits. AI Studio's rate-limit page shows the limits that apply to this project and model. Google doesn't publish a per-tier RPM table anymore, so this signed-in page is the only reliable number.

The audit line is:

API key → project → billing account → plan → tier → balance → monthly cap → rolling spend limit → project rate limits

Change only the layer that failed. A new key in the same project doesn't create a new quota pool or refill an empty balance.

Who owns what: key, project, and billing account

• API key: authenticates a request and belongs to a Google Cloud project. Creating one is free, and it has no plan, tier, or balance of its own. Since May 28, 2026, new AI Studio keys are auth keys bound to a service account and restricted to the Gemini API, and the Gemini API key documentation says the API "rejects requests from unrestricted standard keys."
• Project: groups the usage of its keys. Rate limits "are applied per project, not per API key," according to Google's Gemini API rate-limits documentation.
• Billing account: holds the billing plan, the usage tier, the Prepay balance, and the monthly cap shared by every linked project.

One billing account can stop several projects at once. A $0 balance or a reached monthly cap pauses every project linked to that account together, which is why a "Gemini outage" in one app often shows up in all of them at the same minute.

This applies to the Gemini Developer API billed through AI Studio. If you call Gemini through Vertex AI on Google Cloud, billing follows Google Cloud rather than AI Studio Prepay; see Gemini API vs Vertex AI API for how the two routes differ.

Free and Tier 1–3 are usage tiers, not payment schedules

Usage tiers belong to the billing account. As of October 10, 2026, Google's rate-limits page (last updated October 9, 2026) lists:

Google's rate-limits page listed the Tier 2 limit at $50 on October 5, 2026; it showed $200, the same as Tier 3, when read on November 11, 2026.

Upgrades are automatic and "usually reflect within 10 minutes" once the account qualifies. The 3-day and 30-day clocks start at the first successful payment, not at account creation. Tier 1 alone doesn't mean paid requests will run: on Prepay, the balance must also be above $0. For the Tier 3 path in detail, see Gemini API Tier 3 (T3) Upgrade: $1,000 + 30 Days, Same API Key.

Gemini API free tier limits and what changes on a paid tier

The free tier is per project, and Google doesn't publish a fixed RPM or RPD table for it. Your limits appear on AI Studio's rate-limit page. A Google AI Developers Forum thread titled "Gemini 3.8 Flash Free Tier 20 RPD Is Too Limited for Practical Evaluation" and several September 2026 third-party guides report about 20 requests per day for 3.x Flash models and about 500 for Flash-Lite models. Those are user reports, so trust your own rate-limit page over them.

Three differences matter when you decide whether to add billing, according to Google's Gemini API pricing page:

• Data use. On the free tier, content is "used to improve our products"; on paid tiers it is "not used."
• Models. Gemini 3.8 Flash, 3.6 Flash, 3.5 Flash-Lite, 3.1 Flash-Lite, 3 Flash Preview, the 2.5 models (since September 18, 2026 limited to earlier users), the Live, TTS, and Transcribe models, Embedding 2, and Gemma 4 are free of charge. Gemini 3.1 Pro Preview, Omni Flash, the Nano Banana image models, Veo, and Lyria are paid only.
• Tools. Grounding with Google Search or Maps is not available on the free tier.

The full model list and how free keys work are in the Gemini API Free Tier guide.

Google AI Pro and Ultra are subscriptions for the Gemini app, not API usage tiers. They don't move a billing account to Tier 1 or pay for API calls.

Prepay and Postpay answer “when do I pay?”

The plan decides when you pay. The tier decides how much you can spend.

Prepay details from the billing page that change real decisions:

• Credits expire after 12 months and aren't refundable. Postpay follows the standard Google Cloud refund policy.
• Overshoot carries over. If usage runs past $0 before metering catches up, the service pauses and the negative balance is deducted from your next purchase.
• No automatic fallback. Projects aren't moved back to Free when the balance runs out.
• Credits are locked to the Gemini API. Prepay credits can't pay for other Google Cloud services, and eligible Google Cloud credits are consumed before your Prepay balance.

Two kinds of Google Cloud credit don't pay for the Gemini API at all. Since March 2026, Gemini API usage "is specifically excluded from the $300 Google Cloud Free Trial program," and Welcome credits "can't be used to pay for AI Studio usage." That's how Cloud Billing can show a healthy credit balance while AI Studio shows $0.

Google's launch post for Prepay (spring 2026, first for new US billing accounts) said higher-tier users could later move future usage to standard postpaid billing. Today's billing FAQ says the Prepay-to-Postpay switch is not supported, so plan on Prepay.

Five spend controls that must not be collapsed

"Limit" covers five separate controls. They have different scopes, return different errors, and need different fixes.

Cloud Billing budgets send alerts at thresholds you set. They don't stop traffic, so they aren't on this list.

The rolling spend limit is the one most people don't expect, because it can trip while the balance and monthly cap are both fine. A worked example on Gemini 3.8 Flash at the 2026 rate of $0.75 input / $3.75 output per 1M tokens: a request with 2,000 input tokens and 500 output tokens costs 2,000 × $0.75 / 1,000,000 + 500 × $3.75 / 1,000,000 = $0.003375. Tier 1's $10 window then allows about 10 / 0.003375 ≈ 2,963 such requests in any 10 minutes. At the January 1, 2027 rate, the same request costs $0.00675 and the window allows about 1,481. Output tokens include thinking tokens, so long reasoning shrinks these numbers further.

Requests you can wait on are cheaper through Batch or Flex, which Google prices at half the standard rate. Per-model prices are in Gemini API Pricing 2026: Token Rates, Free Tier, and Key Cost.

Diagnose an interruption from the visible state

Match what you see to the control behind it. The table reflects Google's billing and rate-limit documentation as of October 10, 2026; rows marked "forum report" come from single cases on the Google AI Developers Forum, not from Google.

Errors such as 500, 503, or 504 point to a service problem rather than billing. For 429 retry patterns and how to raise quotas, follow the Gemini API rate-limits guide.

Safe actions for upgrading, controlling spend, or returning to Free

To start or upgrade Paid

Link only the intended project to the intended billing account. On Prepay, buy at least the $5 minimum and confirm a positive balance before sending production traffic. Record the date of the first successful payment, because the Tier 2 and Tier 3 clocks start there. Then read the tier in the Billing Tier column and the active limits on the rate-limit page instead of assuming a receipt grants capacity. Google's rate-limits documentation links a request form for higher limits and says it comes with "no guarantees."

To control spend

Set a project spend cap where it's offered, and leave headroom because it is experimental. Use auto-reload with a monthly auto-charge limit to keep a Prepay balance from hitting $0 at night. Add Cloud Billing budget alerts for early warning. Keep application-side limits on concurrency, output length, and retries too, since none of these dashboards is an instant kill switch.

To return a project to Free

Unlink billing from that project, then confirm the Billing Tier column shows Free and check which models and limits the free tier gives you. An empty Prepay balance doesn't do this for you: Google's billing page says projects aren't moved back to Free automatically.

Recheck these facts before changing production traffic

Billing rules here change on known dates. As of October 10, 2026:

• Postpay cutover: the date in your account notice. Impress Watch reported October 12, 2026.
• Flash introductory prices: Gemini 3.8 Flash and 3.6 Flash stay at $0.75 / $3.75 per 1M tokens through December 31, 2026, then $1.50 / $7.50 from January 1, 2027.
• Credit expiry: each Prepay purchase expires 12 months later.
• Tier table: Google's rate-limits page was last updated October 9, 2026, and the billing page September 28, 2026. Re-read both before you size a balance or a cap.
• Regions: paid and free access both depend on the Gemini API available regions list.

The rule that doesn't change: the key identifies the project, the project links to a billing account, and the billing account supplies the plan, the tier, and the caps shared by every linked project. Diagnose in that order and you won't buy the wrong credit or fix the wrong project.

Gemini API billing and free tier FAQ

Can I switch from Prepay back to Postpay on the Gemini API?

No. Google's billing FAQ says switching from Prepay to Postpay "is not supported." Only Postpay to Prepay is available, through "Switch to Prepay" on the AI Studio Billing page.

What happens to the Gemini API on October 12, 2026 if I do nothing?

If your account is still on Postpay and you haven't switched and bought credits by the cutover date in your notice, paid Gemini API usage is interrupted, according to Impress Watch's report of Google's notice. Free-only accounts aren't affected. Other Google Cloud services on the same billing account stay on Postpay.

Why does Cloud Billing show credits but AI Studio shows $0?

The $300 Google Cloud Free Trial excludes Gemini API usage since March 2026, and Welcome credits can't pay for AI Studio usage. Prepay credits are a separate balance locked to the Gemini API, so AI Studio shows only what you've bought there.

Do unused Gemini API Prepay credits expire?

Yes. Prepay credits expire 12 months after purchase and can't be refunded.

What are the Gemini API free tier limits?

They're set per project and shown on AI Studio's rate-limit page; Google doesn't publish a fixed table. Users report about 20 requests per day for 3.x Flash models and about 500 for Flash-Lite models. The Gemini API Free Tier guide lists the free models and how to read your limits.

Does the Gemini Live API have a free tier?

Yes. Google's pricing page lists the Live models as free of charge on the free tier, alongside TTS and Transcribe. Limits are per project and shown in AI Studio, and free-tier content may be used to improve Google's products.

Is a Gemini API key free, and does it have its own limits?

Creating a key is free, and the key has no limits of its own. Limits and billing belong to the project and its billing account, so several keys in one project share the same quota.

Is Google AI Pro or Ultra the same as a Gemini API tier?

No. Google AI Pro and Ultra are Gemini app subscriptions. API usage tiers come from the billing account linked to your project in AI Studio.