diff --git a/hledger-lib/Hledger/Data/Amount.hs b/hledger-lib/Hledger/Data/Amount.hs
index 279788783..6ceca03d7 100644
--- a/hledger-lib/Hledger/Data/Amount.hs
+++ b/hledger-lib/Hledger/Data/Amount.hs
@@ -91,6 +91,7 @@ module Hledger.Data.Amount (
   showAmountWithoutPrice,
   amountSetPrecision,
   amountSetPrecisionMin,
+  amountSetPrecisionMax,
   withPrecision,
   amountSetFullPrecision,
   amountSetFullPrecisionOr,
@@ -154,6 +155,8 @@ module Hledger.Data.Amount (
   wbUnpack,
   mixedAmountSetPrecision,
   mixedAmountSetFullPrecision,
+  mixedAmountSetPrecisionMin,
+  mixedAmountSetPrecisionMax,
 
   -- * misc.
   tests_Amount
@@ -395,6 +398,12 @@ amountSetPrecisionMin :: Word8 -> Amount -> Amount
 amountSetPrecisionMin minp a = amountSetPrecision p a
   where p = Precision $ max minp (amountDisplayPrecision a)
 
+-- | Ensure an amount's display precision is at most the given maximum precision.
+-- Always sets an explicit Precision.
+amountSetPrecisionMax :: Word8 -> Amount -> Amount
+amountSetPrecisionMax maxp a = amountSetPrecision p a
+  where p = Precision $ min maxp (amountDisplayPrecision a)
+
 -- | Increase an amount's display precision, if needed, to enough decimal places
 -- to show it exactly (showing all significant decimal digits, without trailing zeros).
 -- If the amount's display precision is unset, it will be treated as precision 0.
@@ -1189,6 +1198,16 @@ mixedAmountSetPrecision p = mapMixedAmountUnsafe (amountSetPrecision p)
 mixedAmountSetFullPrecision :: MixedAmount -> MixedAmount
 mixedAmountSetFullPrecision = mapMixedAmountUnsafe amountSetFullPrecision
 
+-- | In each component amount, ensure the display precision is at least the given value.
+-- Makes all amounts have an explicit Precision.
+mixedAmountSetPrecisionMin :: Word8 -> MixedAmount -> MixedAmount
+mixedAmountSetPrecisionMin p = mapMixedAmountUnsafe (amountSetPrecisionMin p)
+
+-- | In each component amount, ensure the display precision is at most the given value.
+-- Makes all amounts have an explicit Precision.
+mixedAmountSetPrecisionMax :: Word8 -> MixedAmount -> MixedAmount
+mixedAmountSetPrecisionMax p = mapMixedAmountUnsafe (amountSetPrecisionMax p)
+
 -- | Remove all prices from a MixedAmount.
 mixedAmountStripPrices :: MixedAmount -> MixedAmount
 mixedAmountStripPrices (Mixed ma) =
diff --git a/hledger-lib/Hledger/Data/Valuation.hs b/hledger-lib/Hledger/Data/Valuation.hs
index c18bc6cc6..b52502498 100644
--- a/hledger-lib/Hledger/Data/Valuation.hs
+++ b/hledger-lib/Hledger/Data/Valuation.hs
@@ -25,6 +25,7 @@ module Hledger.Data.Valuation (
   ,marketPriceReverse
   ,priceDirectiveToMarketPrice
   ,amountPriceDirectiveFromCost
+  ,valuationTypeValuationCommodity
   -- ,priceLookup
   ,tests_Valuation
 )
@@ -68,6 +69,14 @@ data ValuationType =
   | AtDate Day (Maybe CommoditySymbol)  -- ^ convert to default or given valuation commodity, using market prices on some date
   deriving (Show,Eq)
 
+valuationTypeValuationCommodity :: ValuationType -> Maybe CommoditySymbol
+valuationTypeValuationCommodity = \case
+    AtThen   (Just c) -> Just c
+    AtEnd    (Just c) -> Just c
+    AtNow    (Just c) -> Just c
+    AtDate _ (Just c) -> Just c
+    _                 -> Nothing
+
 -- | A price oracle is a magic memoising function that efficiently
 -- looks up market prices (exchange rates) from one commodity to
 -- another (or if unspecified, to a default valuation commodity) on a
@@ -193,17 +202,15 @@ amountValueAtDate priceoracle styles mto d a =
       -- Manage style and precision of the new amount. Initially:
       --  rate is a Decimal with the internal precision of the original market price declaration.
       --  aquantity is a Decimal with a's internal precision.
-      --  The resulting internal precision will be larger than both (their sum ?).
-      --  The display precision will be that of nullamt (0).
-      -- Now apply the standard display style for comm
+      --  The calculated value's internal precision may be different from these.
+      --  Its display precision will be that of nullamt (0).
+      -- Now apply the standard display style for comm (if there is one)
       & styleAmounts styles
-      -- and set the display precision to rate's internal precision
-      -- (unnormalised - don't strip trailing zeros)
-      -- & amountSetPrecision (Precision $ decimalPlaces rate)
-      & amountSetFullPrecisionOr Nothing -- (Just defaultMaxPrecision)
+      -- set the display precision to match the internal precision (showing all digits),
+      -- unnormalised (don't strip trailing zeros);
+      -- but if it looks like an infinite decimal, limit the precision to 8.
+      & amountSetFullPrecisionOr Nothing
       & dbg9With (lbl "calculated value".showAmount)
-      -- & dbg9With (lbl "precision of value".show.amountDisplayPrecision)
-      -- see also print-styles.test, valuation2.test
 
 -- | Calculate the gain of each component amount, that is the difference
 -- between the valued amount and the value of the cost basis (see
diff --git a/hledger/Hledger/Cli/Commands/Roi.hs b/hledger/Hledger/Cli/Commands/Roi.hs
index ec7a239f0..ef083a92a 100644
--- a/hledger/Hledger/Cli/Commands/Roi.hs
+++ b/hledger/Hledger/Cli/Commands/Roi.hs
@@ -65,9 +65,15 @@ roi CliOpts{rawopts_=rawopts, reportspec_=rspec@ReportSpec{_rsReportOpts=ReportO
     -- lbl = lbl_ "roi"
     today = _rsDay rspec
     priceOracle = journalPriceOracle infer_prices_ j
-    styles = journalCommodityStyles j
+    styles = journalCommodityStylesWith HardRounding j
     mixedAmountValue periodlast date =
-        maybe id (mixedAmountApplyValuation priceOracle styles periodlast today date) value_
+        -- These calculations can generate very precise decimals. To avoid showing too many digits:
+        -- If we have no style for the valuation commodity, generate one that will limit the precision ?
+        -- But it's not easy to find out the valuation commodity (or commodities) here if it's implicit,
+        -- as that information is buried in the price graph.
+        -- Instead, do what we don't like to do: hard code a max precision, overriding commodity styles.
+        mixedAmountSetPrecisionMax defaultMaxPrecision
+      . maybe id (mixedAmountApplyValuation priceOracle styles periodlast today date) value_
       . maybe id (mixedAmountToCost styles) conversionop_
 
   let
diff --git a/hledger/test/roi.test b/hledger/test/roi.test
index 3c78c93a2..49afdb787 100644
--- a/hledger/test/roi.test
+++ b/hledger/test/roi.test
@@ -257,7 +257,9 @@ $ hledger -f- roi -p 2019-11 --inv Investment --pnl PnL --cost --value=then,A --
 
 >= 0
 
-# ** 11. Use "then" prices. 10000/76.20 = 131.23, 11000/73.88=148.89
+# ** 11. Use "then" prices. 10000/76.20 = 131.23, 11000/73.88=148.89.
+# Also note that large decimals are limited to 8 digits if there's no
+# standard display precision for them (P directives do not set display precision).
 <
 P 2020-12-01 $ 76.20
 P 2021-01-01 $ 73.88
@@ -269,17 +271,20 @@ P 2021-01-01 $ 73.88
 2021-01-02 get profit
  assets:investment  =11000
  income:investment
+
 $ hledger -f - roi --inv assets:investment --pnl income:investment --value=then,'$'
-+---++------------+------------++---------------+----------+-------------+-----++---------++------------+----------+
-|   ||      Begin |        End || Value (begin) | Cashflow | Value (end) | PnL ||     IRR || TWR/period | TWR/year |
-+===++============+============++===============+==========+=============+=====++=========++============+==========+
-| 1 || 2020-12-02 | 2021-01-02 ||             0 |     $131 |        $149 | $18 || 321.99% ||     13.45% |  323.47% |
-+---++------------+------------++---------------+----------+-------------+-----++---------++------------+----------+
++---++------------+------------++---------------+---------------+---------------+--------------++---------++------------+----------+
+|   ||      Begin |        End || Value (begin) |      Cashflow |   Value (end) |          PnL ||     IRR || TWR/period | TWR/year |
++===++============+============++===============+===============+===============+==============++=========++============+==========+
+| 1 || 2020-12-02 | 2021-01-02 ||             0 | $131.23359580 | $148.89009204 | $17.65649624 || 321.99% ||     13.45% |  323.47% |
++---++------------+------------++---------------+---------------+---------------+--------------++---------++------------+----------+
 
->= 0
+>=
 
-# ** 12. Use "end" prices. 10000/73.88=135.35
+# ** 12. Use "end" prices. 10000/73.88=135.35.
+# And, large decimals can be rounded further with a commodity directive.
 <
+commodity $1000.00
 P 2020-12-01 $ 76.20
 P 2021-01-01 $ 73.88
 
@@ -290,14 +295,15 @@ P 2021-01-01 $ 73.88
 2021-01-02 get profit
  assets:investment  =11000
  income:investment
-$ hledger -f - roi --inv assets:investment --pnl income:investment --value=end,'$'
-+---++------------+------------++---------------+----------+-------------+-----++---------++------------+----------+
-|   ||      Begin |        End || Value (begin) | Cashflow | Value (end) | PnL ||     IRR || TWR/period | TWR/year |
-+===++============+============++===============+==========+=============+=====++=========++============+==========+
-| 1 || 2020-12-02 | 2021-01-02 ||             0 |     $135 |        $149 | $14 || 196.58% ||     10.00% |  197.46% |
-+---++------------+------------++---------------+----------+-------------+-----++---------++------------+----------+
 
->= 0
+$ hledger -f - roi --inv assets:investment --pnl income:investment --value=end,'$'
++---++------------+------------++---------------+----------+-------------+--------++---------++------------+----------+
+|   ||      Begin |        End || Value (begin) | Cashflow | Value (end) |    PnL ||     IRR || TWR/period | TWR/year |
++===++============+============++===============+==========+=============+========++=========++============+==========+
+| 1 || 2020-12-02 | 2021-01-02 ||             0 |  $135.35 |     $148.89 | $13.54 || 196.58% ||     10.00% |  197.46% |
++---++------------+------------++---------------+----------+-------------+--------++---------++------------+----------+
+
+>=
 
 # ** 13. Several PnL transactions on a single date are aggregated together
 <