diff --git a/DESCRIPTION b/DESCRIPTION
index cab3d0d..3babe95 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,5 +1,5 @@
 Package: units
-Version: 0.8-7.0
+Version: 0.8-7.1
 Title: Measurement Units for R Vectors
 Authors@R: c(person("Edzer", "Pebesma", role = c("aut", "cre"), email = "edzer.pebesma@uni-muenster.de", comment = c(ORCID = "0000-0001-8049-7069")),
     person("Thomas", "Mailund", role = "aut", email = "mailund@birc.au.dk"),
diff --git a/NEWS.md b/NEWS.md
index 8a91336..93544e8 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # version devel
 
+* Vectorize `ud_*()` helpers; #405 addressing #404
+
 # version 0.8-7
 
 * Deep copy of `ud_convert()` input to avoid side effects; #403
diff --git a/R/udunits.R b/R/udunits.R
index 9729b0c..3fd09a3 100644
--- a/R/udunits.R
+++ b/R/udunits.R
@@ -3,11 +3,10 @@
 #' Some \pkg{udunits2} utilities are exposed to the user. These functions are
 #' useful for checking whether units are convertible or converting between units
 #' without having to create \pkg{units} objects.
+#' Arguments are recycled if necessary.
 #'
-#' @param from character or object of class \code{symbolic_units},
-#' for the symbol of the original unit.
-#' @param to   character or object of class \code{symbolic_units},
-#' for the symbol of the unit to convert.
+#' @param from,to character vector or object of class \code{symbolic_units},
+#' for the symbol(s) of the original unit(s) and the unit to convert to respectively.
 #' @param ...  unused.
 #'
 #' @return \code{ud_are_convertible}
@@ -23,7 +22,7 @@ ud_are_convertible <- function(from, to, ...) {
     warning("variables `x` and `y` were unfortunate names, and are deprecated",
             "; please use `from` and `to` instead")
   }
-  ud_convertible(ud_char(from), ud_char(to))
+  mapply(ud_convertible, ud_char(from), ud_char(to), USE.NAMES=FALSE)
 }
 
 #' @param x numeric vector
@@ -35,8 +34,8 @@ ud_are_convertible <- function(from, to, ...) {
 #' @export
 #'
 #' @examples
-#' ud_are_convertible("m", "km")
-#' ud_convert(100, "m", "km")
+#' ud_are_convertible(c("m", "mm"), "km")
+#' ud_convert(c(100, 100000), c("m", "mm"), "km")
 #'
 #' a <- set_units(1:3, m/s)
 #' ud_are_convertible(units(a), "km/h")
@@ -45,7 +44,7 @@ ud_are_convertible <- function(from, to, ...) {
 #' ud_are_convertible("degF", "degC")
 #' ud_convert(32, "degF", "degC")
 ud_convert <- function(x, from, to) {
-  ud_convert_doubles(x, ud_char(from), ud_char(to))
+  mapply(ud_convert_doubles, x, ud_char(from), ud_char(to))
 }
 
 ud_char <- function(x) {
diff --git a/man/udunits2.Rd b/man/udunits2.Rd
index 4a31e66..a1f0e37 100644
--- a/man/udunits2.Rd
+++ b/man/udunits2.Rd
@@ -11,11 +11,8 @@ ud_are_convertible(from, to, ...)
 ud_convert(x, from, to)
 }
 \arguments{
-\item{from}{character or object of class \code{symbolic_units},
-for the symbol of the original unit.}
-
-\item{to}{character or object of class \code{symbolic_units},
-for the symbol of the unit to convert.}
+\item{from, to}{character vector or object of class \code{symbolic_units},
+for the symbol(s) of the original unit(s) and the unit to convert to respectively.}
 
 \item{...}{unused.}
 
@@ -33,10 +30,11 @@ returns a numeric vector with \code{x} converted to new unit.
 Some \pkg{udunits2} utilities are exposed to the user. These functions are
 useful for checking whether units are convertible or converting between units
 without having to create \pkg{units} objects.
+Arguments are recycled if necessary.
 }
 \examples{
-ud_are_convertible("m", "km")
-ud_convert(100, "m", "km")
+ud_are_convertible(c("m", "mm"), "km")
+ud_convert(c(100, 100000), c("m", "mm"), "km")
 
 a <- set_units(1:3, m/s)
 ud_are_convertible(units(a), "km/h")
diff --git a/tests/testthat/test_udunits.R b/tests/testthat/test_udunits.R
index 3b5a9a0..4c6fe3d 100644
--- a/tests/testthat/test_udunits.R
+++ b/tests/testthat/test_udunits.R
@@ -4,6 +4,12 @@ test_that("ud_are_convertible return the expected value", {
   expect_true(ud_are_convertible("m", "km"))
   expect_true(ud_are_convertible(units(x), "km"))
   expect_false(ud_are_convertible("s", "kg"))
+
+  x <- c("m", "l")
+  y <- c("km", "ml", "cm", "kg")
+  conv <- c(TRUE, TRUE, TRUE, FALSE)
+  expect_equal(ud_are_convertible(x, y), conv)
+  expect_equal(ud_are_convertible(y, x), conv)
 })
 
 test_that("ud_convert works with simple conversions", {
@@ -16,7 +22,7 @@ test_that("ud_convert works with simple conversions", {
 })
 
 test_that("ud_convert works with vectors", {
-  expect_equal(ud_convert(1:2, "m", "km"), 1:2/1000)
+  expect_equal(ud_convert(1:2, c("m", "mm"), "km"), 1:2/c(1e3,1e6))
   expect_equal(ud_convert(c(32, 212), "degF", "degC"), c(0, 100))
 })