diff --git a/encodings/fsst/src/compute/mod.rs b/encodings/fsst/src/compute/mod.rs
index e23d4272866..8343fc64e00 100644
--- a/encodings/fsst/src/compute/mod.rs
+++ b/encodings/fsst/src/compute/mod.rs
@@ -6,6 +6,7 @@ mod cast;
 mod compare;
 mod filter;
 mod like;
+pub(crate) mod uncompressed_size_in_bytes;
 
 use vortex_array::ArrayRef;
 use vortex_array::ArrayView;
diff --git a/encodings/fsst/src/compute/uncompressed_size_in_bytes.rs b/encodings/fsst/src/compute/uncompressed_size_in_bytes.rs
new file mode 100644
index 00000000000..91dbd46f71b
--- /dev/null
+++ b/encodings/fsst/src/compute/uncompressed_size_in_bytes.rs
@@ -0,0 +1,230 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright the Vortex contributors
+
+use std::mem::size_of;
+
+use vortex_array::ArrayRef;
+use vortex_array::ArrayView;
+use vortex_array::ExecutionCtx;
+use vortex_array::IntoArray;
+use vortex_array::aggregate_fn::AggregateFnRef;
+use vortex_array::aggregate_fn::fns::uncompressed_size_in_bytes::UncompressedSizeInBytes;
+use vortex_array::aggregate_fn::kernels::DynAggregateKernel;
+use vortex_array::arrays::ConstantArray;
+use vortex_array::arrays::PrimitiveArray;
+use vortex_array::arrays::varbinview::BinaryView;
+use vortex_array::match_each_integer_ptype;
+use vortex_array::scalar::Scalar;
+use vortex_error::VortexResult;
+use vortex_error::vortex_err;
+use vortex_mask::Mask;
+
+use crate::FSST;
+use crate::FSSTArraySlotsExt;
+
+/// Computes the canonical FSST size from stored lengths without decoding the string payload.
+///
+/// The stored lengths are authoritative because verifying them requires decoding the payload.
+#[derive(Debug)]
+pub(crate) struct FsstUncompressedSizeKernel;
+
+impl DynAggregateKernel for FsstUncompressedSizeKernel {
+    fn aggregate(
+        &self,
+        aggregate_fn: &AggregateFnRef,
+        batch: &ArrayRef,
+        ctx: &mut ExecutionCtx,
+    ) -> VortexResult<Option<Scalar>> {
+        if !aggregate_fn.is::<UncompressedSizeInBytes>() {
+            return Ok(None);
+        }
+
+        let Some(fsst) = batch.as_opt::<FSST>() else {
+            return Ok(None);
+        };
+
+        Ok(Some(Scalar::from(uncompressed_size(fsst, ctx)?)))
+    }
+}
+
+fn uncompressed_size(fsst: ArrayView<'_, FSST>, ctx: &mut ExecutionCtx) -> VortexResult<u64> {
+    let decoded_bytes = decoded_size(fsst, ctx)?;
+    let validity_size = validity_size(fsst.validity()?.execute_mask(fsst.len(), ctx)?)?;
+
+    canonical_size(fsst.len(), decoded_bytes, validity_size)
+}
+
+fn decoded_size(fsst: ArrayView<'_, FSST>, ctx: &mut ExecutionCtx) -> VortexResult<u64> {
+    let lengths = fsst
+        .uncompressed_lengths()
+        .clone()
+        .execute::<PrimitiveArray>(ctx)?;
+
+    match_each_integer_ptype!(lengths.ptype(), |P| {
+        checked_sum_lengths(lengths.as_slice::<P>())
+    })
+}
+
+fn checked_sum_lengths<P>(lengths: &[P]) -> VortexResult<u64>
+where
+    P: Copy,
+    u64: TryFrom<P>,
+    <u64 as TryFrom<P>>::Error: std::fmt::Display,
+{
+    lengths.iter().try_fold(0u64, |total, length| {
+        let length = u64::try_from(*length).map_err(|error| {
+            vortex_err!("Failed to convert FSST uncompressed length to u64: {error}")
+        })?;
+        total
+            .checked_add(length)
+            .ok_or_else(|| vortex_err!("FSST decoded size overflowed u64"))
+    })
+}
+
+fn validity_size(validity: Mask) -> VortexResult<u64> {
+    match validity {
+        Mask::AllTrue(_) => Ok(0),
+        Mask::AllFalse(length) => Ok(ConstantArray::new(false, length).into_array().nbytes()),
+        Mask::Values(values) => u64::try_from(values.len().div_ceil(u8::BITS as usize))
+            .map_err(|error| vortex_err!("Failed to convert FSST validity size to u64: {error}")),
+    }
+}
+
+fn canonical_size(length: usize, decoded_bytes: u64, validity_bytes: u64) -> VortexResult<u64> {
+    let views_size =
+        u64::try_from(length)
+            .map_err(|error| vortex_err!("Failed to convert FSST length to u64: {error}"))?
+            .checked_mul(u64::try_from(size_of::<BinaryView>()).map_err(|error| {
+                vortex_err!("Failed to convert binary view width to u64: {error}")
+            })?)
+            .ok_or_else(|| vortex_err!("FSST uncompressed size overflowed u64"))?;
+    views_size
+        .checked_add(decoded_bytes)
+        .and_then(|size| size.checked_add(validity_bytes))
+        .ok_or_else(|| vortex_err!("FSST uncompressed size overflowed u64"))
+}
+
+#[cfg(test)]
+mod tests {
+    use rstest::rstest;
+    use vortex_array::ArrayRef;
+    use vortex_array::ArrayVTable;
+    use vortex_array::IntoArray;
+    use vortex_array::VortexSessionExecute;
+    use vortex_array::aggregate_fn::AggregateFnVTable;
+    use vortex_array::aggregate_fn::fns::uncompressed_size_in_bytes::UncompressedSizeInBytes;
+    use vortex_array::aggregate_fn::fns::uncompressed_size_in_bytes::uncompressed_size_in_bytes;
+    use vortex_array::aggregate_fn::session::AggregateFnSessionExt;
+    use vortex_array::array_session;
+    use vortex_array::arrays::VarBinArray;
+    use vortex_array::arrays::VarBinViewArray;
+    use vortex_array::dtype::DType;
+    use vortex_array::dtype::Nullability;
+    use vortex_error::VortexResult;
+
+    use super::checked_sum_lengths;
+    use crate::FSST;
+    use crate::fsst_compress;
+    use crate::fsst_train_compressor;
+
+    fn assert_matches_materialized_size(array: ArrayRef) -> VortexResult<()> {
+        let session = array_session();
+        crate::initialize(&session);
+        let mut ctx = session.create_execution_ctx();
+        let compressor = fsst_train_compressor(&array, &mut ctx)?;
+        let encoded = fsst_compress(&array, &compressor, &mut ctx)?.into_array();
+
+        assert_encoded_size_matches_materialized(&encoded, &mut ctx)
+    }
+
+    fn assert_encoded_size_matches_materialized(
+        encoded: &ArrayRef,
+        ctx: &mut vortex_array::ExecutionCtx,
+    ) -> VortexResult<()> {
+        let actual = u64::try_from(uncompressed_size_in_bytes(encoded, ctx)?)?;
+        let materialized = encoded.clone().execute::<VarBinViewArray>(ctx)?;
+
+        assert_eq!(actual, materialized.as_ref().nbytes());
+        Ok(())
+    }
+
+    #[rstest]
+    #[case::utf8(
+        vec![
+            Some(b"short".to_vec()),
+            Some(b"a string that uses an outlined view".to_vec()),
+            Some(Vec::new()),
+        ],
+        DType::Utf8(Nullability::NonNullable),
+    )]
+    #[case::nullable_utf8(
+        vec![
+            Some(b"alpha".to_vec()),
+            None,
+            Some(b"a nullable outlined string".to_vec()),
+        ],
+        DType::Utf8(Nullability::Nullable),
+    )]
+    #[case::binary(
+        vec![
+            Some(vec![0, 159, 146, 150]),
+            Some(vec![255; 24]),
+            Some(Vec::new()),
+        ],
+        DType::Binary(Nullability::NonNullable),
+    )]
+    #[case::all_invalid(
+        vec![None, None, None],
+        DType::Utf8(Nullability::Nullable),
+    )]
+    fn matches_materialized_size(
+        #[case] values: Vec<Option<Vec<u8>>>,
+        #[case] dtype: DType,
+    ) -> VortexResult<()> {
+        assert_matches_materialized_size(VarBinArray::from_iter(values, dtype).into_array())
+    }
+
+    #[test]
+    fn registers_kernel() {
+        let session = array_session();
+        crate::initialize(&session);
+
+        assert!(
+            session
+                .aggregate_fns()
+                .find_aggregate_kernel(FSST.id(), UncompressedSizeInBytes.id())
+                .is_some()
+        );
+    }
+
+    #[test]
+    fn sliced_nullable_array_matches_materialized_size() -> VortexResult<()> {
+        let session = array_session();
+        crate::initialize(&session);
+        let mut ctx = session.create_execution_ctx();
+        let input = VarBinArray::from_iter(
+            [
+                Some("zero"),
+                None,
+                Some("two uses an outlined string value"),
+                Some("three"),
+                None,
+            ],
+            DType::Utf8(Nullability::Nullable),
+        )
+        .into_array();
+        let compressor = fsst_train_compressor(&input, &mut ctx)?;
+        let encoded = fsst_compress(&input, &compressor, &mut ctx)?
+            .into_array()
+            .slice(1..5)?;
+
+        assert_encoded_size_matches_materialized(&encoded, &mut ctx)
+    }
+
+    #[rstest]
+    #[case::negative(checked_sum_lengths(&[-1i64]))]
+    #[case::overflow(checked_sum_lengths(&[u64::MAX, 1]))]
+    fn rejects_invalid_lengths(#[case] result: VortexResult<u64>) {
+        assert!(result.is_err());
+    }
+}
diff --git a/encodings/fsst/src/lib.rs b/encodings/fsst/src/lib.rs
index 70dcc705249..cc6a62db13a 100644
--- a/encodings/fsst/src/lib.rs
+++ b/encodings/fsst/src/lib.rs
@@ -27,6 +27,10 @@ mod tests;
 
 pub use array::*;
 pub use compress::*;
+use vortex_array::ArrayVTable;
+use vortex_array::aggregate_fn::AggregateFnVTable;
+use vortex_array::aggregate_fn::fns::uncompressed_size_in_bytes::UncompressedSizeInBytes;
+use vortex_array::aggregate_fn::session::AggregateFnSessionExt;
 use vortex_array::session::ArraySessionExt;
 use vortex_session::VortexSession;
 
@@ -34,4 +38,9 @@ use vortex_session::VortexSession;
 pub fn initialize(session: &VortexSession) {
     session.arrays().register(FSST);
     kernel::initialize(session);
+    session.aggregate_fns().register_aggregate_kernel(
+        FSST.id(),
+        Some(UncompressedSizeInBytes.id()),
+        &compute::uncompressed_size_in_bytes::FsstUncompressedSizeKernel,
+    );
 }