diff --git a/api/api_full.go b/api/api_full.go
index 4d65ad23d..f2585b927 100644
--- a/api/api_full.go
+++ b/api/api_full.go
@@ -247,8 +247,8 @@ type FullNode interface {
 	ClientRetrieve(ctx context.Context, order RetrievalOrder, ref *FileRef) (<-chan marketevents.RetrievalEvent, error)
 	// ClientQueryAsk returns a signed StorageAsk from the specified miner.
 	ClientQueryAsk(ctx context.Context, p peer.ID, miner address.Address) (*storagemarket.SignedStorageAsk, error)
-	// ClientCalcCommP calculates the CommP for a specified file, based on the sector size of the provided miner.
-	ClientCalcCommP(ctx context.Context, inpath string, miner address.Address) (*CommPRet, error)
+	// ClientCalcCommP calculates the CommP for a specified file
+	ClientCalcCommP(ctx context.Context, inpath string) (*CommPRet, error)
 	// ClientGenCar generates a CAR file for the specified file.
 	ClientGenCar(ctx context.Context, ref FileRef, outpath string) error
 	// ClientDealSize calculates real deal data size
diff --git a/api/apistruct/struct.go b/api/apistruct/struct.go
index 090f77f82..5be189c98 100644
--- a/api/apistruct/struct.go
+++ b/api/apistruct/struct.go
@@ -137,7 +137,7 @@ type FullNodeStruct struct {
 		ClientListDeals       func(ctx context.Context) ([]api.DealInfo, error)                                                                 `perm:"write"`
 		ClientRetrieve        func(ctx context.Context, order api.RetrievalOrder, ref *api.FileRef) (<-chan marketevents.RetrievalEvent, error) `perm:"admin"`
 		ClientQueryAsk        func(ctx context.Context, p peer.ID, miner address.Address) (*storagemarket.SignedStorageAsk, error)              `perm:"read"`
-		ClientCalcCommP       func(ctx context.Context, inpath string, miner address.Address) (*api.CommPRet, error)                            `perm:"read"`
+		ClientCalcCommP       func(ctx context.Context, inpath string) (*api.CommPRet, error)                                                   `perm:"read"`
 		ClientGenCar          func(ctx context.Context, ref api.FileRef, outpath string) error                                                  `perm:"write"`
 		ClientDealSize        func(ctx context.Context, root cid.Cid) (api.DataSize, error)                                                     `perm:"read"`
 
@@ -426,8 +426,8 @@ func (c *FullNodeStruct) ClientRetrieve(ctx context.Context, order api.Retrieval
 func (c *FullNodeStruct) ClientQueryAsk(ctx context.Context, p peer.ID, miner address.Address) (*storagemarket.SignedStorageAsk, error) {
 	return c.Internal.ClientQueryAsk(ctx, p, miner)
 }
-func (c *FullNodeStruct) ClientCalcCommP(ctx context.Context, inpath string, miner address.Address) (*api.CommPRet, error) {
-	return c.Internal.ClientCalcCommP(ctx, inpath, miner)
+func (c *FullNodeStruct) ClientCalcCommP(ctx context.Context, inpath string) (*api.CommPRet, error) {
+	return c.Internal.ClientCalcCommP(ctx, inpath)
 }
 
 func (c *FullNodeStruct) ClientGenCar(ctx context.Context, ref api.FileRef, outpath string) error {
diff --git a/cli/client.go b/cli/client.go
index 5bc0f4a0d..bfe86e186 100644
--- a/cli/client.go
+++ b/cli/client.go
@@ -174,7 +174,7 @@ var clientDropCmd = &cli.Command{
 var clientCommPCmd = &cli.Command{
 	Name:      "commP",
 	Usage:     "Calculate the piece-cid (commP) of a CAR file",
-	ArgsUsage: "[inputFile minerAddress]",
+	ArgsUsage: "[inputFile]",
 	Flags: []cli.Flag{
 		&CidBaseFlag,
 	},
@@ -186,16 +186,11 @@ var clientCommPCmd = &cli.Command{
 		defer closer()
 		ctx := ReqContext(cctx)
 
-		if cctx.Args().Len() != 2 {
-			return fmt.Errorf("usage: commP <inputPath> <minerAddr>")
+		if cctx.Args().Len() != 1 {
+			return fmt.Errorf("usage: commP <inputPath>")
 		}
 
-		miner, err := address.NewFromString(cctx.Args().Get(1))
-		if err != nil {
-			return err
-		}
-
-		ret, err := api.ClientCalcCommP(ctx, cctx.Args().Get(0), miner)
+		ret, err := api.ClientCalcCommP(ctx, cctx.Args().Get(0))
 		if err != nil {
 			return err
 		}
diff --git a/node/impl/client/client.go b/node/impl/client/client.go
index 2823ca263..404487859 100644
--- a/node/impl/client/client.go
+++ b/node/impl/client/client.go
@@ -565,13 +565,19 @@ func (a *API) ClientQueryAsk(ctx context.Context, p peer.ID, miner address.Addre
 	return signedAsk, nil
 }
 
-func (a *API) ClientCalcCommP(ctx context.Context, inpath string, miner address.Address) (*api.CommPRet, error) {
-	mi, err := a.StateMinerInfo(ctx, miner, types.EmptyTSK)
-	if err != nil {
-		return nil, xerrors.Errorf("failed checking miners sector size: %w", err)
-	}
+func (a *API) ClientCalcCommP(ctx context.Context, inpath string) (*api.CommPRet, error) {
 
-	rt, err := ffiwrapper.SealProofTypeFromSectorSize(mi.SectorSize)
+	// Hard-code the sector size to 32GiB, because:
+	// - pieceio.GeneratePieceCommitment requires a RegisteredSealProof
+	// - commP itself is sector-size independent, with rather low probability of that changing
+	//   ( note how the final rust call is identical for every RegSP type )
+	//   https://github.com/filecoin-project/rust-filecoin-proofs-api/blob/v5.0.0/src/seal.rs#L1040-L1050
+	//
+	// IF/WHEN this changes in the future we will have to be able to calculate
+	// "old style" commP, and thus will need to introduce a version switch or similar
+	arbitrarySectorSize := abi.SectorSize(32 << 30)
+
+	rt, err := ffiwrapper.SealProofTypeFromSectorSize(arbitrarySectorSize)
 	if err != nil {
 		return nil, xerrors.Errorf("bad sector size: %w", err)
 	}