diff --git a/antd/src/grpc/service.rs b/antd/src/grpc/service.rs
index 0e72ae0..4ad4fd5 100644
--- a/antd/src/grpc/service.rs
+++ b/antd/src/grpc/service.rs
@@ -6,7 +6,7 @@ use tonic::{Request, Response, Status};
 
 use crate::error::AntdError;
 use crate::state::AppState;
-use crate::types::format_payment_mode;
+use crate::types::{adjust_for_public_upload, format_payment_mode};
 
 // Generated protobuf modules
 #[allow(dead_code)]
@@ -536,10 +536,16 @@ impl pb::file_service_server::FileService for FileServiceImpl {
         .map_err(AntdError::from_core)
         .map_err(tonic::Status::from)?;
 
+        let (chunk_count, atto_tokens) = if req.is_public {
+            adjust_for_public_upload(estimate.chunk_count, &estimate.storage_cost_atto)
+        } else {
+            (estimate.chunk_count, estimate.storage_cost_atto)
+        };
+
         Ok(Response::new(pb::Cost {
-            atto_tokens: estimate.storage_cost_atto,
+            atto_tokens,
             file_size: estimate.file_size,
-            chunk_count: estimate.chunk_count as u32,
+            chunk_count: chunk_count as u32,
             estimated_gas_cost_wei: estimate.estimated_gas_cost_wei,
             payment_mode: format_payment_mode(estimate.payment_mode),
         }))
diff --git a/antd/src/rest/files.rs b/antd/src/rest/files.rs
index 73ca3a9..245aeb9 100644
--- a/antd/src/rest/files.rs
+++ b/antd/src/rest/files.rs
@@ -218,10 +218,16 @@ pub async fn file_cost(
     .map_err(|e| AntdError::Internal(format!("task failed: {e}")))?
     .map_err(AntdError::from_core)?;
 
+    let (chunk_count, cost) = if req.is_public {
+        adjust_for_public_upload(estimate.chunk_count, &estimate.storage_cost_atto)
+    } else {
+        (estimate.chunk_count, estimate.storage_cost_atto)
+    };
+
     Ok(Json(CostResponse {
-        cost: estimate.storage_cost_atto,
+        cost,
         file_size: estimate.file_size,
-        chunk_count: estimate.chunk_count,
+        chunk_count,
         estimated_gas_cost_wei: estimate.estimated_gas_cost_wei,
         payment_mode: format_payment_mode(estimate.payment_mode),
     }))
diff --git a/antd/src/types.rs b/antd/src/types.rs
index e363bc8..ef21d3a 100644
--- a/antd/src/types.rs
+++ b/antd/src/types.rs
@@ -225,7 +225,6 @@ pub struct CostResponse {
 #[derive(Deserialize)]
 pub struct FileCostRequest {
     pub path: String,
-    #[allow(dead_code)]
     #[serde(default = "default_true")]
     pub is_public: bool,
 }
@@ -234,6 +233,27 @@ fn default_true() -> bool {
     true
 }
 
+/// Adjust a chunk count + storage cost estimate to reflect a public upload,
+/// which bundles one additional DataMap chunk into the same payment batch.
+///
+/// Returns `(adjusted_chunk_count, adjusted_storage_cost_atto)`. If the input
+/// cost cannot be parsed, only the chunk count is bumped and the cost is
+/// returned unchanged.
+pub fn adjust_for_public_upload(chunk_count: usize, storage_cost_atto: &str) -> (usize, String) {
+    let new_chunk_count = chunk_count.saturating_add(1);
+    if chunk_count == 0 {
+        return (new_chunk_count, storage_cost_atto.to_string());
+    }
+    let total: ant_core::data::U256 = match storage_cost_atto.parse() {
+        Ok(v) => v,
+        Err(_) => return (new_chunk_count, storage_cost_atto.to_string()),
+    };
+    let divisor = ant_core::data::U256::from(chunk_count as u64);
+    let per_chunk = total / divisor;
+    let new_total = total + per_chunk;
+    (new_chunk_count, new_total.to_string())
+}
+
 /// Parse a payment mode string into ant-core's PaymentMode.
 pub fn parse_payment_mode(mode: Option<&str>) -> Result<ant_core::data::PaymentMode, String> {
     match mode {
@@ -451,4 +471,59 @@ mod tests {
         assert_eq!(json["payment_token_address"], "");
         assert_eq!(json["payment_vault_address"], "");
     }
+
+    #[test]
+    fn adjust_for_public_upload_bumps_count_and_scales_cost() {
+        // 5 chunks, 1000 atto total → per-chunk = 200 atto.
+        // Public upload adds 1 chunk → 6 chunks, 1200 atto total.
+        let (chunks, cost) = adjust_for_public_upload(5, "1000");
+        assert_eq!(chunks, 6);
+        assert_eq!(cost, "1200");
+    }
+
+    #[test]
+    fn adjust_for_public_upload_handles_uneven_division() {
+        // 3 chunks, 100 atto total → per-chunk = 33 atto (integer divide).
+        // Result: 4 chunks, 133 atto. Slight rounding is acceptable for
+        // an estimate; exact pricing is the on-chain quote.
+        let (chunks, cost) = adjust_for_public_upload(3, "100");
+        assert_eq!(chunks, 4);
+        assert_eq!(cost, "133");
+    }
+
+    #[test]
+    fn adjust_for_public_upload_handles_large_atto_values() {
+        // Real-world atto costs frequently exceed u64. Verify U256 handles it.
+        // 10 chunks at 1e22 atto total = 10K ANT — well above u64::MAX.
+        let total = "10000000000000000000000"; // 1e22
+        let (chunks, cost) = adjust_for_public_upload(10, total);
+        assert_eq!(chunks, 11);
+        // 1e22 + 1e21 = 1.1e22
+        assert_eq!(cost, "11000000000000000000000");
+    }
+
+    #[test]
+    fn adjust_for_public_upload_zero_chunks_only_bumps_count() {
+        // Defensive: shouldn't happen in practice, but division-by-zero would.
+        let (chunks, cost) = adjust_for_public_upload(0, "0");
+        assert_eq!(chunks, 1);
+        assert_eq!(cost, "0");
+    }
+
+    #[test]
+    fn adjust_for_public_upload_unparseable_cost_only_bumps_count() {
+        // ant-core returns "0" for already-stored chunks; a non-numeric or
+        // negative value would be a contract bug, but stay graceful.
+        let (chunks, cost) = adjust_for_public_upload(5, "not-a-number");
+        assert_eq!(chunks, 6);
+        assert_eq!(cost, "not-a-number");
+    }
+
+    #[test]
+    fn adjust_for_public_upload_zero_cost_round_trips() {
+        // Already-stored case: cost stays 0, count still bumps.
+        let (chunks, cost) = adjust_for_public_upload(5, "0");
+        assert_eq!(chunks, 6);
+        assert_eq!(cost, "0");
+    }
 }